Process for Alkali Metal-Selenium Secondary Battery Containing a Cathode of Encapsulated Selenium Particles

ABSTRACT

Provided is a method of manufacturing an alkali metal-selenium cell, comprising: (a) providing a cathode; (b) providing an alkali metal anode; and (c) combining the anode and the cathode and adding an electrolyte in ionic contact with the anode and the cathode to form the cell; wherein the cathode contains multiple particulates of a selenium-containing material selected from selenium, a selenium-carbon hybrid, selenium-graphite hybrid, selenium-graphene hybrid, conducting polymer-selenium hybrid, a metal selenide, a Se alloy or mixture with Sn, Sb, Bi, S, or Te, a selenium compound, or a combination thereof and wherein at least one of the particulates comprises one or a plurality of selenium-containing material particles being embraced or encapsulated by a thin layer of an elastomer having a recoverable tensile strain from 5% to 1000%, a lithium ion conductivity no less than 10 −7  S/cm, and a thickness from 0.5 nm to 10 μm.

FIELD OF THE INVENTION

The present invention is related to a unique cathode composition andcathode structure in a secondary or rechargeable alkali metal-seleniumbattery, including the lithium-selenium battery, sodium-seleniumbattery, and potassium-selenium battery, and a method of producing same.

BACKGROUND

Rechargeable lithium-ion (Li-ion) and lithium metal batteries (includingLi-sulfur, Li-selenium, and Li metal-air batteries) are consideredpromising power sources for electric vehicle (EV), hybrid electricvehicle (HEV), and portable electronic devices, such as lap-topcomputers and mobile phones. Lithium as a metal element has the highestcapacity (3,861 mAh/g) compared to any other metal or metal-intercalatedcompound as an anode active material (except Li_(4.4)Si, which has aspecific capacity of 4,200 mAh/g). Hence, in general, Li metal batterieshave a significantly higher energy density than lithium ion batteries.

Historically, rechargeable lithium metal batteries were produced usingnon-lithiated compounds having relatively high specific capacities, suchas TiS₂, MoS₂, MnO₂, CoO₂, and V₂O₅, as the cathode active materials,which were coupled with a lithium metal anode. When the battery wasdischarged, lithium ions were transferred from the lithium metal anodethrough the electrolyte to the cathode, and the cathode becamelithiated. Unfortunately, upon repeated charges/discharges, the lithiummetal resulted in the formation of dendrites at the anode thatultimately grew to penetrate through the separator, causing internalshorting and explosion. As a result of a series of accidents associatedwith this problem, the production of these types of secondary batterieswas stopped in the early 1990's, giving ways to lithium-ion batteries.In lithium-ion batteries, pure lithium metal sheet or film was replacedby carbonaceous materials as the anode. The carbonaceous materialabsorbs lithium (through intercalation of lithium ions or atoms betweengraphene planes, for instance) and desorbs lithium ions during there-charge and discharge phases, respectively, of the lithium ion batteryoperation. The carbonaceous material may comprise primarily graphitethat can be intercalated with lithium and the resulting graphiteintercalation compound may be expressed as Li_(x)C₆, where x istypically less than 1.

Although lithium-ion (Li-ion) batteries are promising energy storagedevices for electric drive vehicles, state-of-the-art Li-ion batterieshave yet to meet the cost and performance targets. Li-ion cellstypically use a lithium transition-metal oxide or phosphate as apositive electrode (cathode) that de/re-intercalates Li⁺ at a highpotential with respect to the carbon negative electrode (anode). Thespecific capacity of lithium transition-metal oxide or phosphate basedcathode active material is typically in the range of 150-180 mAh/g. As aresult, the specific energy of commercially available Li-ion cells istypically in the range of 120-240 Wh/kg, most. These specific energyvalues are two to three times lower than what would be required forbattery-powered electric vehicles to be widely accepted.

With the rapid development of hybrid (HEV), plug-in hybrid electricvehicles (HEV), and all-battery electric vehicles (EV), there is anurgent need for anode and cathode materials that provide a rechargeablebattery with a significantly higher specific energy, higher energydensity, higher rate capability, long cycle life, and safety. Two of themost promising energy storage devices are the lithium-sulfur (Li—S) celland lithium-selenium (Li—Se) cell since the theoretical capacity of Liis 3,861 mAh/g, that of S is 1,675 mAh/g, and that of Se is 675 mAh/g.Compared with conventional intercalation-based Li-ion batteries, Li—Sand Li—Se cells have the opportunity to provide a significantly higherenergy density (a product of capacity and voltage). Having asignificantly higher electronic conductivity as compared to S, Se is amore effective cathode active material and, as such, Li—Se potentiallycan exhibit a higher rate capability.

However, Li—Se cell is still plagued with several major technicalproblems that have hindered its widespread commercialization:

-   -   (1) All prior art Li—Se cells have dendrite formation and        related internal shorting issues;        (2) The cell tends to exhibit significant capacity decay during        discharge-charge cycling. This is mainly due to the high        solubility of selenium and lithium polyselenide anions formed as        reaction intermediates during both discharge and charge        processes in the polar organic solvents used in electrolytes.        During cycling, the anions can migrate through the separator to        the Li negative electrode whereupon they are reduced to solid        precipitates and cannot return to the cathode, causing active        mass loss. This phenomenon is commonly referred to as the        Shuttle Effect. This process leads to several problems: high        self-discharge rates, loss of cathode capacity, corrosion of        current collectors and electrical leads leading to loss of        electrical contact to active cell components, fouling of the        anode surface giving rise to malfunction of the anode, and        clogging of the pores in the cell membrane separator which leads        to loss of ion transport and large increases in internal        resistance in the cell.        (3) Presumably, nanostructured mesoporous carbon materials could        be used to hold the Se or lithium polyselenide in their pores,        preventing large out-flux of these species from the porous        carbon structure through the electrolyte into the anode.        However, the fabrication of the proposed highly ordered        mesoporous carbon structure requires a tedious and expensive        template-assisted process. It is also challenging to load a        large proportion of selenium into the mesoscaled pores of these        materials using a physical vapor deposition or solution        precipitation process. Typically the maximum loading of Se in        these porous carbon structures is less than 50% by weight (i.e.        the amount of active material is less than 50%; more than 50%        being inactive materials).

Despite the various approaches proposed for the fabrication of highenergy density Li—Se cells, there remains a need for cathode materials,production processes, and cell operation methods that retard theout-diffusion of Se or lithium polyselenide from the cathodecompartments into other components in these cells, improve theutilization of electro-active cathode materials (Se utilizationefficiency), and provide rechargeable Li—Se cells with high capacitiesover a large number of cycles.

Most significantly, lithium metal (including pure lithium, lithiumalloys of high lithium content with other metal elements, orlithium-containing compounds with a high lithium content; e.g. >80% orpreferably >90% by weight Li) still provides the highest anode specificcapacity as compared to essentially all other anode active materials(except pure silicon, but silicon has pulverization issues). Lithiummetal would be an ideal anode material in a lithium-selenium secondarybattery if dendrite related issues could be addressed.

Sodium metal (Na) and potassium metal (K) have similar chemicalcharacteristics to Li and the selenium cathode in sodium-selenium cells(Na—Se batteries) or potassium-selenium cells (K—Se) face the sameissues observed in Li—S batteries, such as: (i) low active materialutilization rate, (ii) poor cycle life, and (iii) low Coulumbicefficiency. Again, these drawbacks arise mainly from insulating natureof Se, dissolution of polyselenide intermediates in liquid electrolytes(and related Shuttle effect), and large volume change during repeatedcharges/discharges.

Hence, an object of the present invention is to provide a rechargeableLi—Se battery that exhibits an exceptionally high specific energy orhigh energy density. One particular technical goal of the presentinvention is to provide a Li metal-selenium or Li ion-selenium cell witha cell specific energy greater than 300 Wh/Kg, preferably greater than350 Wh/Kg, and more preferably greater than 400 Wh/Kg (all based on thetotal cell weight).

It may be noted that in most of the open literature reports (scientificpapers) and patent documents, scientists or inventors choose to expressthe cathode specific capacity based on the selenium or lithiumpolyselenide weight alone (not the total cathode composite weight), butunfortunately a large proportion of non-active materials (those notcapable of storing lithium, such as conductive additive and binder) istypically used in their Li—Se cells. For practical use purposes, it ismore meaningful to use the cathode composite weight-based capacityvalue.

A specific object of the present invention is to provide a rechargeablelithium-selenium or sodium-selenium cell based on rational materials andbattery designs that overcome or significantly reduce the followingissues commonly associated with conventional Li—Se and Na—Se cells: (a)dendrite formation (internal shorting); (b) low electric and ionicconductivities of selenium, requiring large proportion (typically30-55%) of non-active conductive fillers and having significantproportion of non-accessible or non-reachable selenium, or lithium orsodium polyselenide); (c) dissolution of lithium polyselenide or sodiumpolyselenide in electrolyte and migration of dissolved lithium/sodiumpolyselenide from the cathode to the anode (which irreversibly reactwith lithium/sodium at the anode), resulting in active material loss andcapacity decay (the shuttle effect); and (d) short cycle life.

SUMMARY OF THE INVENTION

The present invention provides an alkali metal-selenium cell (e.g.lithium-selenium cell, sodium-selenium cell, and potassium-seleniumcell). The alkali metal-selenium cell comprises (a) an anode activematerial layer and an optional anode current collector supporting theanode active material layer; (b) a cathode active material layer and anoptional cathode current collector supporting the cathode activematerial layer; and (c) an electrolyte with an optional porous separatorlayer in ionic contact with the anode active material layer and thecathode active material layer; wherein the cathode active material layercontains multiple particulates of a selenium-containing materialselected from a selenium-carbon hybrid, selenium-graphite hybrid,selenium-graphene hybrid, conducting polymer-selenium hybrid, a metalsulfide, a Se alloy or mixture with Sn, Sb, Bi, S, or Te, a seleniumcompound, or a combination thereof and wherein at least one of theparticulates is composed of one or a plurality of theselenium-containing material particles being embraced or encapsulated bya thin layer of an elastomer having a recoverable tensile strain no lessthan 5% (typically from 5% to 1000%) when measured without an additiveor reinforcement, a lithium ion conductivity no less than 10⁻⁷ S/cm(typically from 10⁻⁵ S/cm to 5×10⁻² S/cm, measured at room temperature),and a thickness from 0.5 nm to 10 μm (typically from 1 nm to 1 μm, butpreferably <100 nm and more preferably <10 nm).

The selenium-carbon hybrid, selenium-graphite hybrid, selenium-graphenehybrid, or conducting polymer-selenium hybrid may be a mixture, blend,composite, or chemically or physically bonded entity of selenium orsulfide with a carbon, graphite, graphene, or conducting polymermaterial. For instance, a selenium-graphene hybrid can be a simplemixture (in a particle form) of selenium and graphene prepared byball-milling. Such a hybrid can contain selenium bonded on surfaces of agraphene oxide sheet, etc. As another example, the selenium-carbonhybrid can be a simple mixture (in a particle form) of selenium andcarbon nanotubes, or can contain selenium residing in pores of activatedcarbon particles.

In some embodiments, the elastomer (also referred to as an elastomericmaterial) contains a material selected from natural polyisoprene (e.g.cis-1,4-polyisoprene natural rubber (NR) and trans-1,4-polyisoprenegutta-percha), synthetic polyisoprene (IR for isoprene rubber),polybutadiene (BR for butadiene rubber), chloroprene rubber (CR),polychloroprene (e.g. Neoprene, Baypren etc.), butyl rubber (copolymerof isobutylene and isoprene, IIR), including halogenated butyl rubbers(chloro butyl rubber (CIIR) and bromo butyl rubber (BIIR),styrene-butadiene rubber (copolymer of styrene and butadiene, SBR),nitrile rubber (copolymer of butadiene and acrylonitrile, NBR), EPM(ethylene propylene rubber, a copolymer of ethylene and propylene), EPDMrubber (ethylene propylene diene rubber, a terpolymer of ethylene,propylene and a diene-component), epichlorohydrin rubber (ECO),polyacrylic rubber (ACM, ABR), silicone rubber (SI, Q, VMQ),fluorosilicone rubber (FVMQ), fluoroelastomers (FKM, and FEPM; such asViton, Tecnoflon, Fluorel, Aflas and Dai-El), perfluoroelastomers (FFKM:

Tecnoflon PFR, Kalrez, Chemraz, Perlast), polyether block amides (PEBA),chlorosulfonated polyethylene (CSM; e.g. Hypalon), and ethylene-vinylacetate (EVA), thermoplastic elastomers (TPE), protein resilin, proteinelastin, ethylene oxide-epichlorohydrin copolymer, polyurethane,urethane-urea copolymer, and combinations thereof.

In the rechargeable alkali metal-selenium cell, the metal sulfide maycontain a material denoted by M_(x)Se_(y), wherein x is an integer from1 to 3 and y is an integer from 1 to 10, and M is a metal elementselected from an alkali metal, an alkaline metal selected from Mg or Ca,a transition metal, a metal from groups 13 to 17 of the periodic table,or a combination thereof. The metal element M preferably is selectedfrom Li, Na, K, Mg, Zn, Cu, Ti, Ni, Co, Fe, or Al.

In some preferred embodiments, the metal sulfide in the cathode layercontains Li₂Se₁, Li₂Se₂, Li₂Se₃, Li₂Se₄, Li₂Se₅, Li₂Se₆, Li₂Se₇, Li₂Se₈,Li₂Se₉, Li₂Se₁₀, Na₂Se₁, Na₂Se₂, Na₂Se₃, Na₂Se₄, Na₂Se₅, Na₂Se₆, Na₂Se₇,Na₂Se₈, Na₂Se₉, Na₂Se₁₀, K₂Se₁, K₂Se₂, K₂Se₃, K₂Se₄, K₂Se₅, K₂Se₆,K₂Se₇, K₂Se₈, K₂Se₉, or K₂Se₁₀.

In the rechargeable alkali metal-selenium cell, the carbon or graphitematerial in the cathode active material layer may be selected frommesophase pitch, mesophase carbon, mesocarbon micro-bead (MCMB), cokeparticle, expanded graphite flake, artificial graphite particle, naturalgraphite particle, highly oriented pyrolytic graphite, soft carbonparticle, hard carbon particle, carbon nanotube, carbon nanofiber,carbon fiber, graphite nanofiber, graphite fiber, carbonized polymerfiber, activated carbon, carbon black, or a combination thereof. Thegraphene may be selected from pristine graphene, graphene oxide, reducedgraphene oxide (RGO), graphene fluoride, nitrogenated graphene,hydrogenated graphene, doped graphene, functionalized graphene, or acombination thereof.

The conducting polymer-selenium hybrid may preferably contain anintrinsically conductive polymer selected from polyaniline, polypyrrole,polythiophene, polyfuran, a bicyclic polymer, a sulfonated derivativethereof, or a combination thereof.

In certain embodiments, the elastomer contains from 0.1% to 50% byweight of a lithium ion-conducting additive dispersed therein, orcontains therein from 0.1% by weight to 10% by weight of a reinforcementnanofilament selected from carbon nanotube, carbon nanofiber, graphene,or a combination thereof.

In certain embodiments, the elastomer is mixed with a lithiumion-conducting additive to form a composite wherein the lithiumion-conducting additive is dispersed in the elastomer and is selectedfrom Li₂CO₃, Li₂O, Li₂C₂O₄, LiOH, LiX, ROCO₂Li, HCOLi, ROLi, (ROCO₂Li)₂,(CH₂OCO₂Li)₂, Li₂S, Li_(x)SO_(y), or a combination thereof, wherein X═F,Cl, I, or Br, R=a hydrocarbon group, 0<x≤1 and 1≤y≤4.

In certain embodiments, the elastomer is mixed with a lithiumion-conducting additive to form a composite wherein the lithiumion-conducting additive is dispersed in the elastomer and is selectedfrom lithium perchlorate (LiClO₄), lithium hexafluorophosphate (LiPF₆),lithium borofluoride (LiBF₄), lithium hexafluoroarsenide (LiAsF₆),lithium trifluoro-metasulfonate (LiCF₃SO₃), bis-trifluoromethylsulfonylimide lithium (LiN(CF₃SO₂)₂), lithium bis(oxalato)borate(LiBOB), lithium oxalyldifluoroborate (LiBF₂C₂O₄), lithiumoxalyldifluoroborate (LiBF₂C₂O₄), lithium nitrate (LiNO₃),Li-fluoroalkyl-phosphate (LiPF₃(CF₂CF₃)₃), lithiumbisperfluoro-ethysulfonylimide (LiBETI), lithiumbis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide,lithium trifluoromethanesulfonimide (LiTFSI), an ionic liquid-basedlithium salt, or a combination thereof.

In certain embodiments, the elastomer is mixed with a sodiumion-conducting additive to form a composite. The sodium-conductingadditive may be selected from, for example, Na₂CO₃, Na₂O, Na₂C₂O₄, NaOH,NaX, ROCO₂Na, HCONa, RONa, (ROCO₂Na)₂, (CH₂OCO₂Na)₂, Na₂S, Na_(x)SO_(y),or a combination thereof, wherein X═F, Cl, I, or Br, R=a hydrocarbongroup, 0<x≤1 and 1≤y≤4.

The sodium-conducting additive also may be selected from sodiumperchlorate (NaClO₄), sodium hexafluorophosphate (NaPF₆), sodiumborofluoride (NaBF₄), sodium hexafluoroarsenide (NaAsF₆), sodiumtrifluoro-metasulfonate (NaCF₃SO₃), bis-trifluoromethyl sulfonylimidesodium (NaN(CF₃ SO₂)₂), sodium bis(oxalato)borate (NaBOB), sodiumoxalyldifluoroborate (NaBF₂C₂O₄), sodium oxalyldifluoroborate(NaBF₂C₂O₄), sodium nitrate (NaNO₃), Na-fluoroalkyl-phosphates(NaPF₃(CF₂CF₃)₃), sodium bisperfluoro-ethysulfonylimide (NaBETI), sodiumbis(trifluoromethanesulfonyl)imide, sodium bis(fluorosulfonyl)imide,sodium trifluoromethanesulfonimide (NaTFSI), an ionic liquid-basedsodium salt, or a combination thereof.

In certain preferred embodiments, the elastomer is mixed with anelectron-conducting polymer selected from polyaniline, polypyrrole,polythiophene, polyfuran, a bi-cyclic polymer, a sulfonated derivativethereof, or a combination thereof.

In certain embodiments, the elastomer forms a mixture or blend with alithium ion-conducting polymer selected from poly(ethylene oxide) (PEO),polypropylene oxide (PPO), poly(acrylonitrile) (PAN), poly(methylmethacrylate) (PMMA), poly(vinylidene fluoride) (PVDF), poly bis-methoxyethoxyethoxide-phosphazene, polyvinyl chloride, polydimethylsiloxane,poly(vinylidene fluoride)-hexafluoropropylene (PVDF-HFP), a sulfonatedderivative thereof, or a combination thereof.

Typically, the elastomer has a lithium ion conductivity or sodium ionconductivity from 1×10⁻⁵ S/cm to 5×10⁻² S/cm at room temperature.

The rechargeable alkali metal-selenium cell has a selenium utilizationefficiency from 80% to 99%, more typically from 85% to 97%.

In the rechargeable alkali metal-selenium cell, the electrolyte isselected from polymer electrolyte, polymer gel electrolyte, compositeelectrolyte, ionic liquid electrolyte, non-aqueous liquid electrolyte,soft matter phase electrolyte, solid-state electrolyte, or a combinationthereof. The electrolyte may contain a salt selected from lithiumperchlorate (LiClO₄), lithium hexafluorophosphate (LiPF₆), lithiumborofluoride (LiBF₄), lithium hexafluoroarsenide (LiAsF₆), lithiumtrifluoro-metasulfonate (LiCF₃ SO₃), bis-trifluoromethyl sulfonylimidelithium (LiN(CF₃SO₂)₂, lithium bis(oxalato)borate (LiBOB), lithiumoxalyldifluoroborate (LiBF₂C₂O₄), lithium oxalyldifluoroborate(LiBF₂C₂O₄), lithium nitrate (LiNO₃), Li-fluoroalkyl-phosphates(LiPF3(CF₂CF₃)₃), lithium bisperfluoroethysulfonylimide (LiBETI), anionic liquid salt, sodium perchlorate (NaClO₄), potassium perchlorate(KClO₄), sodium hexafluorophosphate (NaPF₆), potassiumhexafluorophosphate (KPF₆), sodium borofluoride (NaBF₄), potassiumborofluoride (KBF₄), sodium hexafluoroarsenide, potassiumhexafluoroarsenide, sodium trifluoro-metasulfonate (NaCF₃SO₃), potassiumtrifluoro-metasulfonate (KCF₃SO₃), bis-trifluoromethyl sulfonylimidesodium (NaN(CF₃SO₂)₂), sodium trifluoromethanesulfonimide (NaTFSI),bis-trifluoromethyl sulfonylimide potassium (KN(CF₃ SO₂)₂), or acombination thereof.

The solvent may be selected from ethylene carbonate (EC), dimethylcarbonate (DMC), methylethyl carbonate (MEC), diethyl carbonate (DEC),ethyl propionate, methyl propionate, propylene carbonate (PC),gamma.-butyrolactone (γ-BL), acetonitrile (AN), ethyl acetate (EA),propyl formate (PF), methyl formate (MF), toluene, xylene or methylacetate (MA), fluoroethylene carbonate (FEC), vinylene carbonate (VC),allyl ethyl carbonate (AEC), 1,3-dioxolane (DOL), 1,2-dimethoxyethane(DME), tetraethylene glycol dimethylether (TEGDME), poly(ethyleneglycol) dimethyl ether (PEGDME), diethylene glycol dibutyl ether(DEGDBE), 2-ethoxyethyl ether (EEE), sulfone, sulfolane, roomtemperature ionic liquid, or a combination thereof.

In certain embodiments, the anode active material layer contains ananode active material selected from lithium metal, sodium metal,potassium metal, a lithium metal alloy, sodium metal alloy, potassiummetal alloy, a lithium intercalation compound, a sodium intercalationcompound, a potassium intercalation compound, a lithiated compound, asodiated compound, a potassium-doped compound, lithiated titaniumdioxide, lithium titanate, lithium manganate, a lithium transition metaloxide, Li₄Ti₅O₁₂, or a combination thereof.

The rechargeable alkali metal-selenium cell may be a lithiumion-selenium cell and, in this case, the anode active material layercontains an anode active material selected from the group consisting of:(a) silicon (Si), germanium (Ge), tin (Sn), lead (Pb), antimony (Sb),bismuth (Bi), zinc (Zn), aluminum (Al), nickel (Ni), cobalt (Co),manganese (Mn), titanium (Ti), iron (Fe), and cadmium (Cd), andlithiated versions thereof; (b) alloys or intermetallic compounds of Si,Ge, Sn, Pb, Sb, Bi, Zn, Al, or Cd with other elements, and lithiatedversions thereof, wherein said alloys or compounds are stoichiometric ornon-stoichiometric; (c) oxides, carbides, nitrides, sulfides,phosphides, selenides, and tellurides of Si, Ge, Sn, Pb, Sb, Bi, Zn, Al,Fe, Ni, Co, Ti, Mn, or Cd, and their mixtures or composites, andlithiated versions thereof; (d) salts and hydroxides of Sn and lithiatedversions thereof; (e) carbon or graphite materials and prelithiatedversions thereof; and combinations thereof.

The rechargeable alkali metal-selenium cell may be a sodium ion-seleniumcell or potassium ion-selenium cell and, in this case, the anode activematerial layer contains an anode active material selected from the groupconsisting of: (a) sodium- or potassium-doped silicon (Si), germanium(Ge), tin (Sn), lead (Pb), antimony (Sb), bismuth (Bi), zinc (Zn),aluminum (Al), titanium (Ti), cobalt (Co), nickel (Ni), manganese (Mn),cadmium (Cd), and mixtures thereof; (b) sodium- or potassium-containingalloys or intermetallic compounds of Si, Ge, Sn, Pb, Sb, Bi, Zn, Al, Ti,Co, Ni, Mn, Cd, and their mixtures; (c) sodium- or potassium-containingoxides, carbides, nitrides, sulfides, phosphides, selenides, tellurides,or antimonides of Si, Ge, Sn, Pb, Sb, Bi, Zn, Al, Fe, Ti, Co, Ni, Mn,Cd, and mixtures or composites thereof; (d) Sodium or potassium salts;(e) particles of graphite, hard carbon, soft carbon or carbon particlesand pre-sodiated versions thereof and combinations thereof.

Preferably, in the rechargeable alkali metal-selenium cell, theparticulates contain from 80% to 99% by weight of selenium, metalselenide, or metal compound based on the total weight of thehigh-capacity polymer and the selenium, metal selenide, or metalcompound combined.

The present invention also provides a cathode active material layer fora rechargeable alkali metal-selenium cell. This cathode active materiallayer contains multiple particulates of a selenium-containing materialselected from a selenium-carbon hybrid, selenium-graphite hybrid,selenium-graphene hybrid, conducting polymer-selenium hybrid, a metalselenide, a Se alloy or mixture with Sn, Sb, Bi, S, or Te, a seleniumcompound, or a combination thereof and wherein at least one of saidparticulates is composed of one or a plurality of selenium-containingmaterial particles being embraced or encapsulated by a thin layer of aelastomer having a recoverable tensile strain no less than 10% whenmeasured without an additive or reinforcement, a lithium ionconductivity no less than 10⁻⁷ S/cm at room temperature (typically up to5×10⁻² S/cm), and a thickness from 0.5 nm to 10 μm (preferably andtypically from 1 nm to 1 μm, more preferably <100 nm).

In this product (a cathode layer), the selenium-carbon hybrid,selenium-graphite hybrid, selenium-graphene hybrid, or conductingpolymer-selenium hybrid is a mixture, blend, composite, chemically orphysically bonded entity of selenium or selenide with a carbon,graphite, graphene, or conducting polymer material.

In this cathode active material layer product, the elastomer preferablycontains a material selected from natural polyisoprene, syntheticpolyisoprene, polybutadiene, chloroprene rubber, polychloroprene, butylrubber, styrene-butadiene rubber, nitrile rubber, ethylene propylenerubber, ethylene propylene diene rubber, epichlorohydrin rubber,polyacrylic rubber, silicone rubber, fluorosilicone rubber,perfluoroelastomers, polyether block amides, chlorosulfonatedpolyethylene, ethylene-vinyl acetate, thermoplastic elastomer, proteinresilin, protein elastin, ethylene oxide-epichlorohydrin copolymer,polyurethane, urethane-urea copolymer, or a combination thereof.

In the cathode active material layer, the metal selenide may containM_(x)Se_(y), wherein x is an integer from 1 to 3 and y is an integerfrom 1 to 10, and M is a metal element selected from an alkali metal, analkaline metal selected from Mg or Ca, a transition metal, a metal fromgroups 13 to 17 of the periodic table, or a combination thereof.

The carbon or graphite material in the cathode active material layer maybe selected from mesophase pitch, mesophase carbon, mesocarbon microbead(MCMB), coke particle, expanded graphite flake, artificial graphiteparticle, natural graphite particle, highly oriented pyrolytic graphite,soft carbon particle, hard carbon particle, carbon nanotube, carbonnanofiber, carbon fiber, graphite nanofiber, graphite fiber, carbonizedpolymer fiber, activated carbon, carbon black, or a combination thereof.

This cathode active material layer further comprises a binder resin thatbonds the multiple particulates (of encapsulated selenium-containingparticles) together to form the cathode active material layer, whereinthe binder resin is not part of the multiple particulates (i.e. notincluded inside the core portion of a particulate) and is external tothe multiple particulates. In other words, the elastomer does notembrace the binder resin.

In the alternative, the present invention also provides a cathode activematerial layer for a rechargeable alkali metal-selenium cell, whereinthe cathode active material layer contains a resin binder, an optionalconductive additive, and multiple particles of a selenium-containingmaterial selected from a selenium-carbon hybrid, selenium-graphitehybrid, selenium-graphene hybrid, conducting polymer-selenium hybrid, ametal selenide, a Se alloy or mixture with Sn, Sb, Bi, S, or Te, aselenium compound, or a combination thereof, wherein theselenium-containing material particles are bonded by the resin binder toform an integral solid layer (a layer of adequate structural integrityso that it can be freely-standing), and wherein the integral solid layeris covered and protected by a thin layer of a elastomer having arecoverable tensile strain no less than 5% when measured without anadditive or reinforcement, a lithium ion conductivity no less than 10⁻⁵S/cm at room temperature, and a thickness from 0.5 nm to 10 μm. In someembodiments, the integral solid layer is bonded by the resin binder to acathode current collector.

Such an elastomer protective layer can be formed by spraying theprecursor mass (monomer or oligomer with the required initiator orcuring agent) over a pre-made cathode active material layer and thenpolymerized and cross-linked.

The invention also provides a rechargeable alkali metal-selenium cellthat contains such a cathode active material layer protected by anelastomer. This alkali metal-selenium cell comprises: (a) an anodeactive material layer and an optional anode current collector supportingthe anode active material layer; (b) a cathode that contains thiscathode active material layer; and (c) an electrolyte with an optionalporous separator layer in ionic contact with the anode active materiallayer and the cathode active material layer.

The present invention also provides a powder mass product for use in alithium-selenium battery cathode. The powder mass comprises multipleparticulates of a selenium-containing material selected from aselenium-carbon hybrid, selenium-graphite hybrid, selenium-graphenehybrid, conducting polymer-selenium hybrid, a metal selenide, a Se alloyor mixture with Sn, Sb, Bi, S, or Te, a selenium compound, or acombination thereof and wherein at least one of the particulatescomprises one or a plurality of selenium-containing material particlesbeing embraced or encapsulated by a thin layer of a elastomer having arecoverable tensile strain no less than 5% when measured without anadditive or reinforcement, a lithium ion conductivity no less than 10⁻⁷S/cm at room temperature, and a thickness from 0.5 nm to 10 μm.

In the powder mass, the selenium-carbon hybrid, selenium-graphitehybrid, selenium-graphene hybrid, or conducting polymer-selenium hybridis a mixture, blend, composite, chemically or physically bonded entityof selenium or selenide with a carbon, graphite, graphene, or conductingpolymer material. The elastomer contains a material selected fromnatural polyisoprene, synthetic polyisoprene, polybutadiene, chloroprenerubber, polychloroprene, butyl rubber, styrene-butadiene rubber, nitrilerubber, ethylene propylene rubber, ethylene propylene diene rubber,epichlorohydrin rubber, polyacrylic rubber, silicone rubber,fluorosilicone rubber, perfluoroelastomers, polyether block amides,chlorosulfonated polyethylene, ethylene-vinyl acetate, thermoplasticelastomer, protein resilin, protein elastin, ethyleneoxide-epichlorohydrin copolymer, polyurethane, urethane-urea copolymer,or a combination thereof.

In the powder mass, the metal selenide preferably contains M_(x)Se_(y),wherein x is an integer from 1 to 3 and y is an integer from 1 to 10,and M is a metal element selected from an alkali metal, an alkalinemetal selected from Mg or Ca, a transition metal, a metal from groups 13to 17 of the periodic table, or a combination thereof.

The invention also provides a method of manufacturing a powder mass fora lithium-selenium battery cathode, the method comprising (a) dispersingparticles of a selenium-containing material in an elastomer solution toform a slurry, wherein the selenium-containing material is selected froma selenium-carbon hybrid, selenium-graphite hybrid, selenium-graphenehybrid, conducting polymer-selenium hybrid, metal selenide, a Se alloyor mixture with Sn, Sb, Bi, S, or Te, a selenium compound, or acombination thereof; and (b) dispensing and forming the slurry into saidpowder mass comprising multiple particulates of the selenium-containingmaterial wherein at least one of the particulates comprises one or aplurality of selenium-containing material particles being embraced orencapsulated by a thin layer of an elastomer having a recoverabletensile strain no less than 5% when measured without an additive orreinforcement, a lithium ion conductivity no less than 10⁻⁷ S/cm at roomtemperature, and a thickness from 0.5 nm to 10 μm.

Preferably, the step of dispensing and forming comprises a procedureselected from pan coating, air suspension, centrifugal extrusion,vibrational nozzle, spray-drying, ultrasonic spraying,coacervation-phase separation, interfacial polycondensation, in-situpolymerization, matrix polymerization, or a combination thereof. Theelastomer solution may contain a precursor monomer or oligomer to anelastomer or an uncured polymer dissolved in a liquid solvent (e.g.water or organic solvent). The step of dispensing and forming caninclude a procedure of removing (drying) the liquid solvent,polymerizing the monomer or oligomer, and/or curing/cross-linking thepolymer.

The present invention also provides a method of manufacturing arechargeable alkali metal-selenium cell. The method comprises: (a)providing a cathode and an optional cathode current collector to supportthe cathode; (b) providing an alkali metal anode, selected from Li, Na,K, or a combination thereof and an optional anode current collector tosupport the anode; (c) combining the anode and the cathode and adding anelectrolyte in contact with the anode and the cathode to form the alkalimetal-selenium cell; wherein the cathode contains multiple particulatesof a selenium-containing material wherein at least one of theparticulates is composed of one or a plurality of selenium-containingmaterial particles which are embraced or encapsulated by a thin layer ofa elastomer having a recoverable tensile strain from 5% to 700% whenmeasured without an additive or reinforcement (more typically from 10%to 300%), a lithium ion conductivity no less than 10⁻⁷ S/cm (typicallyfrom 1×10⁻⁵ S/cm to 5×10⁻² S/cm) at room temperature, and a thicknessfrom 0.5 nm to 10 μm (preferably from 1 nm to 1 more preferably from 1nm to 100 nm, and most preferably, from 1 nm to 10 nm). A separator maybe added to electrically separate the anode and the cathode if theelectrolyte is not a solid electrolyte.

In the above manufacturing method, the selenium-containing materialpreferably is selected from a selenium-carbon hybrid, selenium-graphitehybrid, selenium-graphene hybrid, conducting polymer-selenium hybrid, ametal selenide, a Se alloy or mixture with Sn, Sb, Bi, S, or Te, aselenium compound, or a combination thereof. The selenium-carbon hybrid,selenium-graphite hybrid, selenium-graphene hybrid, or conductingpolymer-selenium hybrid is a mixture, blend, composite, chemically orphysically bonded entity of selenium or selenide with a carbon,graphite, graphene, or conducting polymer material

In the invented manufacturing method, the elastomer preferably containsa material selected from natural polyisoprene, synthetic polyisoprene,polybutadiene, chloroprene rubber, polychloroprene, butyl rubber,styrene-butadiene rubber, nitrile rubber, ethylene propylene rubber,ethylene propylene diene rubber, epichlorohydrin rubber, polyacrylicrubber, silicone rubber, fluorosilicone rubber, perfluoroelastomers,polyether block amides, chlorosulfonated polyethylene, ethylene-vinylacetate, thermoplastic elastomer, protein resilin, protein elastin,ethylene oxide-epichlorohydrin copolymer, polyurethane, urethane-ureacopolymer, or a combination thereof.

In the manufacturing method, the operation of providing multipleparticulates may include encapsulating or embracing the one or aplurality of selenium-containing material particles with a thin layer ofelastomer using a procedure selected from pan coating, air suspension,centrifugal extrusion, vibrational nozzle, spray-drying, ultrasonicspraying, coacervation-phase separation, interfacial polycondensation,in-situ polymerization, matrix polymerization, or a combination thereof.

In some embodiments, the operation of providing multiple particulatesincludes encapsulating or embracing said one or a plurality ofselenium-containing material particles with a mixture of said elastomerwith an elastomer, an electronically conductive polymer, a lithium-ionconducting material, a sodium ion conducting additive, a reinforcementmaterial, or a combination thereof. Preferably, the lithiumion-conducting material is dispersed in said elastomer and is selectedfrom Li₂CO₃, Li₂O, Li₂C₂O₄, LiOH, LiX, ROCO₂Li, HCOLi, ROLi, (ROCO₂Li)₂,(CH₂OCO₂Li)₂, Li₂S, Li_(x)SO_(y), or a combination thereof, wherein X═F,Cl, I, or Br, R=a hydrocarbon group, 0<x≤1 and 1≤y≤4.

In certain embodiments, the lithium ion-conducting material is dispersedin said elastomer and is selected from lithium perchlorate (LiClO₄),lithium hexafluorophosphate (LiPF₆), lithium borofluoride (LiBF₄),lithium hexafluoroarsenide (LiAsF₆), lithium trifluoro-metasulfonate(LiCF₃SO₃), bis-trifluoromethyl sulfonylimide lithium (LiN(CF₃SO₂)₂),lithium bis(oxalato)borate (LiBOB), lithium oxalyldifluoroborate(LiBF₂C₂O₄), lithium oxalyldifluoroborate (LiBF₂C₂O₄), lithium nitrate(LiNO₃), Li-fluoroalkyl-phosphate (LiPF₃(CF₂CF₃)₃), lithiumbisperfluoro-ethysulfonylimide (LiBETI), lithiumbis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide,lithium trifluoromethanesulfonimide (LiTFSI), an ionic liquid-basedlithium salt, or a combination thereof.

In the instant Li—Se cell, the reversible specific capacity of theselenium cathode is typically and preferably no less than 500 mAh pergram and often exceeds 600 or even 625 mAh per gram of entire cathodelayer. The high specific capacity of the presently invented cathode,when combined with a lithium anode, typically leads to a cell specificenergy significantly greater than 350 Wh/Kg, based on the total cellweight including anode, cathode, electrolyte, separator, and currentcollector weights combined. This specific energy value is not based onthe cathode active material weight or cathode layer weight only (assometimes did in open literature or patent applications); instead, thisis based on entire cell weight. In many cases, the cell specific energyis higher than 400 Wh/Kg and, in some examples, exceeds 450 Wh/kg.

These and other advantages and features of the present invention willbecome more transparent with the description of the following best modepractice and illustrative examples.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(A) Schematic of a prior art lithium or sodium metal-seleniumbattery cell, wherein the anode layer is a thin coating or foil of ananode active material (Li or Na metal) and the cathode is composed ofparticles of a cathode active material, a conductive additive (notshown) and a resin binder (not shown).

FIG. 1(B) Schematic of a prior art lithium-ion selenium battery; theanode layer being composed of particles of an anode active material(e.g. fully lithiated Si particles), a conductive additive (not shown)and a resin binder (not shown).

FIG. 2 Schematic illustrating the notion that expansion/shrinkage ofelectrode active material particles, upon lithium insertion andde-insertion during discharge/charge of a prior art lithium-ion battery,can lead to detachment of resin binder from the particles, interruptionof the conductive paths formed by the conductive additive, and loss ofcontact with the current collector.

FIG. 3 Schematic of the presently invented elastomer-encapsulatedcathode active material particles (e.g. Se or Li₂Se particles). The highelasticity of the elastomer shell enables the shell to expand andcontract congruently and conformingly with the core particle.

FIG. 4 Several different types of particulates containingelastomer-encapsulated cathode active material particles (e.g. particlesof Se, lithium polyselenide, sodium polyselenide, potassiumpolyselenide, a Se alloy or mixture with Sn, Sb, Bi, S, or Te, or acombination thereof).

FIG. 5 The specific discharge capacity values of three Li—Se batteryhaving a Se/CNT cathode active material featuring (1) SBR-encapsulatedSe/CNT particles, (2) carbon-encapsulated Se/CNT particles, and (3)un-protected Se/CNT particles, respectively.

FIG. 6 The cycling behaviors of 2 Li—Se cells: one cell has a cathodecontaining particulates of polyurethane-encapsulated selenium-CNTcomposite balls and the other cell has a cathode containing particulatesof un-protected selenium-CNT composite balls.

FIG. 7 The specific discharge capacity values of two Li—Se cells havinga cathode active material layer featuring (1) urethane-ureacopolymer-encapsulated, selenium-MCMB (activated) composite particles;and (2) un-protected selenium-MCMB (activated) composite particles,respectively.

FIG. 8 Ragone plots (cell power density vs. cell energy density) of twoLi metal-selenium cells: one featuring a cathode layer composed ofelastomer-encapsulated Se nanowires and the other a cathode of carbonblack-Se nanoparticles ball-milled together.

FIG. 9 Ragone plots (cell power density vs. cell energy density) of fouralkali metal-selenium cells: Na—Se cell featuringelastomer/RGO-encapsulated particles of selenium, Na—Se cell featuringcarbon-coated Se particles, K—Se cell featuringelastomer/RGO-encapsulated Se particles, and K—Se cell featuringpolyaniline-coated Se particles.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

For convenience, the following discussion of preferred embodiments isprimarily based on Li—Se cells, but the same or similar composition,structure, and methods are applicable to Na—Se and K—Se cells. Examplesare presented for Li—Se cells, Na—Se cells, and K—Se cells.

A. Alkali Metal-Selenium Cells (Using Lithium-Selenium Cells as anExample)

The specific capacity and specific energy of a Li—Se cell (or Na—Se, orK—Se cell) are dictated by the actual amount of selenium that can beimplemented in the cathode active layer (relative to other non-activeingredients, such as the binder resin and conductive filler) and theutilization rate of this selenium amount (i.e. the utilizationefficiency of the cathode active material or the actual proportion of Sethat actively participates in storing and releasing lithium ions). UsingLi—Se cell as an illustrative example, a high-capacity and high-energyLi—Se cell requires a high amount of Se in the cathode active layer(i.e. relative to the amounts of non-active materials, such as thebinder resin, conductive additive, and other modifying or supportingmaterials) and a high Se utilization efficiency). The present inventionprovides such a cathode active layer, its constituent powder massproduct, the resulting Li—Se cell, and a method of producing such acathode active layer and battery.

The alkali metal-selenium cell comprises (a) an anode active materiallayer and an optional anode current collector supporting the anodeactive material layer; (b) a cathode active material layer and anoptional cathode current collector supporting the cathode activematerial layer; and (c) an electrolyte with an optional porous separatorlayer in ionic contact with the anode active material layer and thecathode active material layer; wherein the cathode active material layercontains multiple particulates of a selenium-containing materialselected from a selenium-carbon hybrid, selenium-graphite hybrid,selenium-graphene hybrid, conducting polymer-selenium hybrid, a metalselenide, a Se alloy or mixture with Sn, Sb, Bi, S, or Te, a seleniumcompound, or a combination thereof and wherein at least one of theparticulates is composed of one or a plurality of theselenium-containing material particles being embraced or encapsulated bya thin layer of a elastomer having a recoverable tensile strain no lessthan 10% when measured without an additive or reinforcement, a lithiumion conductivity or sodium ion conductivity no less than 10⁻⁷ S/cm(typically from 10⁻⁵ S/cm to 5×10⁻² S/cm, measured at room temperature),and a thickness from 0.5 nm to 10 μm (typically from 1 nm to 1 μm, butpreferably <100 nm and more preferably <10 nm).

The selenium-carbon hybrid, selenium-graphite hybrid, selenium-graphenehybrid, or conducting polymer-selenium hybrid may be a mixture, blend,composite, or chemically or physically bonded entity of selenium orselenide with a carbon, graphite, graphene, or conducting polymermaterial. For instance, a selenium-graphene hybrid can be a simplemixture (in a particle form) of selenium and graphene prepared byball-milling. Such a hybrid can contain selenium bonded on surfaces of agraphene oxide sheet, etc. As another example, the selenium-carbonhybrid can be a simple mixture (in a particle form) of selenium andcarbon nanotubes, or can contain selenium residing in pores of activatedcarbon particles.

In the invented rechargeable alkali metal-selenium cell, the elastomermay contain a cross-linked network of polymer chains having an etherlinkage, nitrile-derived linkage, benzo peroxide-derived linkage,ethylene oxide linkage, propylene oxide linkage, vinyl alcohol linkage,cyano-resin linkage, triacrylate monomer-derived linkage, tetraacrylatemonomer-derived linkage, or a combination thereof in said cross-linkednetwork of polymer chains. In some preferred embodiments, the elastomercontains a cross-linked network of polymer chains selected fromnitrile-containing polyvinyl alcohol chains, cyanoresin chains,pentaerythritol tetraacrylate chains, pentaerythritol triacrylatechains, ethoxylated trimethylolpropane triacrylate (ETPTA) chains,ethylene glycol methyl ether acrylate (EGMEA) chains, or a combinationthereof.

In the rechargeable alkali metal-selenium cell, the metal selenide maycontain a material denoted by M_(x)Se_(y), wherein x is an integer from1 to 3 and y is an integer from 1 to 10, and M is a metal elementselected from an alkali metal, an alkaline metal selected from Mg or Ca,a transition metal, a metal from groups 13 to 17 of the periodic table,or a combination thereof. The metal element M preferably is selectedfrom Li, Na, K, Mg, Zn, Cu, Ti, Ni, Co, Fe, or Al. In some preferredembodiments, the metal selenide in the cathode layer contains Li₂Se₁,Li₂Se₂, Li₂Se₃, Li₂Se₄, Li₂Se₅, Li₂Se₆, Li₂Se₇, Li₂Se₈, Li₂Se₉, Li₂Se₁₀,Na₂Se₁, Na₂Se₂, Na₂Se₃, Na₂Se₄, Na₂Se₅, Na₂Se₆, Na₂Se₇, Na₂Se₈, Na₂Se₉,Na₂Se₁₀, K₂Se₁, K₂Se₂, K₂Se₃, K₂Se₄, K₂Se₅, K₂Se₆, K₂Se₇, K₂Se₈, K₂Se₉,or K₂Se₁₀.

In the rechargeable alkali metal-selenium cell, the carbon or graphitematerial in the cathode active material layer may be selected frommesophase pitch, mesophase carbon, mesocarbon micro-bead (MCMB), cokeparticle, expanded graphite flake, artificial graphite particle, naturalgraphite particle, highly oriented pyrolytic graphite, soft carbonparticle, hard carbon particle, carbon nanotube, carbon nanofiber,carbon fiber, graphite nanofiber, graphite fiber, carbonized polymerfiber, activated carbon, carbon black, or a combination thereof. Thegraphene may be selected from pristine graphene, graphene oxide, reducedgraphene oxide (RGO), graphene fluoride, nitrogenated graphene,hydrogenated graphene, doped graphene, functionalized graphene, or acombination thereof.

The conducting polymer-selenium hybrid may preferably contain anintrinsically conductive polymer selected from polyaniline, polypyrrole,polythiophene, polyfuran, a bicyclic polymer, a sulfonated derivativethereof, or a combination thereof. This can be a simple mixture ofselenium or metal selenide with a conducting polymer.

In certain embodiments, the elastomer contains from 0.1% to 50% byweight of a lithium ion-conducting additive dispersed therein, orcontains therein from 0.1% by weight to 10% by weight of a reinforcementnanofilament selected from carbon nanotube, carbon nanofiber, graphene,or a combination thereof. The lithium ion-conducting additive to form acomposite wherein said lithium ion-conducting additive is dispersed insaid elastomer and is selected from Li₂CO₃, Li₂O, Li₂C₂O₄, LiOH, LiX,ROCO₂Li, HCOLi, ROLi, (ROCO₂Li)₂, (CH₂OCO₂Li)₂, Li₂S, Li_(x)SO_(y), or acombination thereof, wherein X═F, Cl, I, or Br, R=a hydrocarbon group,0<x≤1 and 1≤y≤4.

The lithium ion-conducting additive may be dispersed in the elastomerand may be selected from lithium perchlorate (LiClO₄), lithiumhexafluorophosphate (LiPF₆), lithium borofluoride (LiBF₄), lithiumhexafluoroarsenide (LiAsF₆), lithium trifluoro-metasulfonate (LiCF₃SO₃),bis-trifluoromethyl sulfonylimide lithium (LiN(CF₃SO₂)₂), lithiumbis(oxalato)borate (LiBOB), lithium oxalyldifluoroborate (LiBF₂C₂O₄),lithium oxalyldifluoroborate (LiBF₂C₂O₄), lithium nitrate (LiNO₃),Li-fluoroalkyl-phosphate (LiPF₃(CF₂CF₃)₃), lithiumbisperfluoro-ethysulfonylimide (LiBETI), lithiumbis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide,lithium trifluoromethanesulfonimide (LiTFSI), an ionic liquid-basedlithium salt, or a combination thereof.

Elastomer refers to a polymer, typically a lightly cross-linked polymer,which exhibits an elastic deformation that is at least 5% when measured(without an additive or reinforcement in the polymer) under uniaxialtension. In the field of materials science and engineering, the “elasticdeformation” is defined as a deformation of a material (when beingmechanically stressed) that is essentially fully recoverable and therecovery is essentially instantaneous upon release of the load. Theelastic deformation is preferably greater than 10%, more preferablygreater than 50%, further more preferably greater than 100%, still morepreferably greater than 150%, and most preferably greater than 200%. Thepreferred types of high-capacity polymers will be discussed later.

As illustrated in FIG. 4, the present invention provides four majortypes of particulates of high-capacity polymer-encapsulated cathodeactive material particles. The first one is a single-particleparticulate containing a cathode active material core 10 (e.g. particleof a selenium-CNT mixture) encapsulated by a high-capacity polymer shell12. The second is a multiple-particle particulate containing multiplecathode active material particles 14 (e.g. particles ofselenium-graphene mixture, selenium-carbon black mixture, activatedcarbon particles having pores impregnated with S, lithium polyselenideparticles, etc.), optionally along with other active materials (e.g.particles of graphite or hard carbon, not shown) or conductive additive,which are encapsulated by a high-capacity polymer 16. The third is asingle-particle particulate containing a cathode active material core 18coated by a carbon or graphene layer 20 (or other conductive material)and further encapsulated by a elastomer 22. The fourth is amultiple-particle particulate containing multiple cathode activematerial particles 24 coated with a conductive protection layer 26(carbon, graphene, etc.), optionally along with other active materials(e.g. particles of graphite or hard carbon, not shown) or conductiveadditive, which are encapsulated by a elastomer shell 28. These cathodeactive material particles can be based on selenium compound, metalpolyselenide, etc., instead of neat selenium.

As schematically illustrated in the upper portion of FIG. 3, aselenium-based particle can be encapsulated by a high-capacity polymershell to form a core-shell structure (selenium core and polymer shell inthis example). As the lithium-selenium battery is discharged, thecathode active material (e.g. selenium in the high-capacitypolymer-encapsulated Se/CNT particle) reacts with lithium ions to formlithium polyselenide which expands in volume. Due to the high elasticityof the encapsulating shell (the high-capacity polymer), the shell willnot be broken into segments (in contrast to the broken carbon shell).The high-capacity polymer shell remains intact, preventing the exposureof the underlying lithium selenide to electrolyte and, thus, preventingthe lithium selenide from dissolving in the electrolyte during repeatedcharges/discharges of the battery. This strategy prevents continuedmigration of lithium polyselenide to the anode side which reacts withlithium and is unable to return to the cathode (the shuttle effect).This shuttle effect is mainly responsible for continued capacity decayin a conventional Li—Se, Na—Se, or K—Se cell.

Alternatively, referring to the lower portion of FIG. 3, lithiumselenide is used as the cathode active material. A layer ofhigh-capacity polymer may be encapsulated around the lithiumpolyselenide particle to form a core-shell structure. When the Li—Sebattery is charged and lithium ions are released from the cathode, thecathode active material particle contracts. However, the high-capacitypolymer is capable of elastically shrinking in a conformal manner;hence, leaving behind no gap between the protective shell and theselenium. Such a configuration is amenable to subsequent lithiumreaction with selenium. The high-capacity polymer shell expands andshrinks congruently with the expansion and shrinkage of the encapsulatedcore cathode active material particle, enabling long-term cyclingstability of a lithium battery.

Production of Se particles, from nanometer to micron scales, is wellknown in the art and fine Se powders are commercially available.Micron-scaled Se particles are easily produced using ball-milling if theinitial powder size is too big. Due to the low melting point (221° C.)of Se, one can easily obtain Se melt and use a melt atomizationtechnique to produce sub-micron Se particles, for instance. Variousmethods have been used in the past for synthesizing Se nanoparticle(SeNP), such as chemical reduction method, biological synthesis,solvothermal route, hydrothermal route, microwave assisted synthesis,green synthesis, electrodeposition method, and pulsed laser ablationmethod. The following references may be consulted for the details ofseveral methods of producing SeNP:

-   1. Sheng-Yi Zhang, Juan Zhang, Hong-Yan Wang, Hong-Yuan Chen,    “Synthesis of selenium nanoparticles in the presence of    polysaccharides,” Materials Letters, Volume 58, Issue 21, August    2004, Pages 2590-2594-   2. Urarika Luesakul, Seamkwan Komenek, Songchan Puthong, Nongnuj    Muangsin, “Shape-controlled synthesis of cubic-like selenium    nanoparticles via the self-assembly method,” Carbohydrate Polymers,    Volume 153, 20 Nov. 2016, Pages 435-444.-   3. C. Dwivedi, et al., “An Organic Acid-Induced Synthesis and    Characterization of Selenium Nanoparticles,” Journal of    Nanotechnology, 2011: 1-6.-   4. Lin, Z., Lin, F. and Wang, C.R.C. “Observation in the Growth of    Selenium Nanoparticles,” Journal of Chinese Chemical Society, 2004,    51 (2): 239-242.-   5. Gao, B. X., Zhang, J. and Zhang, L., “Hollow Sphere Selenium    Nanoparticles: Their In-Vitro Anti Hydroxyl Radical Effect,”    Advanced Materials, 14 (4), (2002) 290-293.-   6. Li, Z. and Hua, P. 2009. “Mixed Surfactant Template Method for    Preparation of Nanometer Selenium,” E-Journal of Chemistry    6 (1) (2009) 304-310.-   7. Chen, H., Shin, D., Nam, J., Kwon, K. and Yoo, J. 2010. “Selenium    Nanowiresand Nanotubes Synthesized via a Facile Template-Free    Solution Method,” Materials Research Bulletin 45 (6) (2010)    699-704.)-   8. Zeng, K., Chen, S., Song, Y., Li, H., Li, F. and Liu, P. 2013,    “Solvothermal Synthesis of Trigonal Selenium with Butterfly-like    Microstructure,” Particuology, 11 (5) (2013) 614-617.)-   9. An, C. and Wang, S. 2007. “Diameter-Selected Synthesis of Single    Crystalline Trigonal Selenium Nanowires.∥Materials Chemistry and    Physics, 2007, 101 (2-3): 357-361.-   10. An, C., Tang, K., Liu, X. and Qian, Y., “Large-Scale Synthesis    of High Quality Trigonal Selenium Nanowires.∥European Journal of    Inorganic Chemistry,” 2003 (17): 3250-3255.

For instance, the chemical reduction method employs reduction ofselenium salt using variety of reducing agents such as surfactants andbiocompatible chemicals to obtain stabilized colloidal suspensions ofnanoparticles. Various shapes and sizes of SeNP are synthesized usingthese methods. Chemical reduction method assists in maintaining betteruniformity of the particles.

Dwivedi et al. [Ref. 3] used carboxylic acids like acetic acid, oxalicacid and aromatic acid (gallic acid) to synthesize SeNP of sphericalshape and size 40-100 nm using sodium selenosulfate as the source ofselenium. Lin et al. [Ref 4] used sulfur dioxide and SDS as reducingagents and selenous acid was used as a precursor to synthesize SeNP witha size range of 30-200 nm. Gao et al. [Ref. 5] used β-mercaptoethanol asa reducing agent producing hollow sphere SeNP (HSSN) of size 32 nm.

A mixed surfactant synthesis carried out by Li and Hua [Ref 6] showedthe use of dihydroascorbic acid with sodium dodecyl sulfate andpolyvinyl chloride to prepare SeNP of size 30 nm. A study reported byChen et al. [Ref 7] used template free solution to prepare trigonalNanowires and Nanotubes of 70-100 nm width and 180-350 nm respectivelywherein, glucose was selected as a reducing agent and sodium selenite asthe selenium source forming α-Se. Recrystallization of these SeNPwithout template or a surfactant resulted in the transformation of α-Seto t-Se.

The solvothermal or hydrothermal method employs usage of a solvent underhigh pressure and temperature that involves the interaction ofprecursors during synthesis. For instance, Zeng et al. [Ref. 8]synthesized nanoparticles using this method wherein, selenium wasdissolved in ethylenediamine and kept in a Teflon coated autoclavemaintaining the temperature at 160° C. for 2 hour and then cooled to RTto form a brown homogenous solution and then acetone stored at −18° C.was added to this solution to make it amorphous SeNP and furthertransforming it into trigonal selenium of hexagonal rod shapedstructure. These particles on aging acquired a butterfly-likemicrostructure having 4 μm in width and 8 μm in length.

A study conducted by An & Wang [Ref. 9 and 10] showed synthesis oftrigonal selenium Nanowires of 10-60 nm in size using sodium seleniteand thiosulphate salts as starting materials. Steam under pressure wasused for the synthesis with a set temperature of 180° C.

Once the particles of Se are produced, they can be incorporated into apolymer-liquid medium suspension to make a polymer mixture suspension,dispersion or slurry. This suspension, dispersion, or slurry is thensubjected to secondary particle formation treatment, such asspray-drying, spray-pyrolysis, ultrasonic spraying, and vibration-nozzledroplet formation, to make the invented polymer-protected particulates.

B. Elastomers

Preferably and typically, the elastomer has a lithium ion conductivityno less than 10⁻⁷ S/cm, more preferably no less than 10⁻⁴ S/cm, furtherpreferably no less than 10⁻³ S/cm, and most preferably no less than 10⁻²S/cm. In some embodiments, the elastomer is a neat polymer having noadditive or filler dispersed therein. In others, the elastomer is anelastomer matrix composite containing from 0.1% to 50% (preferably 1% to35%) by weight of a lithium ion-conducting additive dispersed in anelastomer matrix material. The elastomer must have a high elasticity(elastic deformation strain value >5%). An elastic deformation is adeformation that is fully recoverable and the recovery process isessentially instantaneous (no significant time delay). The elastomer canexhibit an elastic deformation from 5% up to 1,000% (10 times of itsoriginal length), more typically from 10% to 800%, and further moretypically from 30% to 300%, and most typically and desirably from 70% to300%. It may be noted that although a metal typically has a highductility (i.e. can be extended to a large extent without breakage), themajority of the deformation is plastic deformation (non-recoverable,permanent deformation) and only a small amount of elastic deformation(typically <1% and more typically <0.2%).

In some preferred embodiments, the elastomer contains a materialselected from natural polyisoprene, synthetic polyisoprene,polybutadiene, chloroprene rubber, polychloroprene, butyl rubber,styrene-butadiene rubber, nitrile rubber, ethylene propylene rubber,ethylene propylene diene rubber, epichlorohydrin rubber, polyacrylicrubber, silicone rubber, fluorosilicone rubber, perfluoroelastomers,polyether block amides, chlorosulfonated polyethylene, ethylene-vinylacetate, thermoplastic elastomer, protein resilin, protein elastin,ethylene oxide-epichlorohydrin copolymer, polyurethane, urethane-ureacopolymer, or a combination thereof.

Typically, an elastomer is originally in a monomer or oligomer statesthat can be cured to form a cross-linked polymer that is highly elastic.Prior to curing, these polymers or oligomers are soluble in an organicsolvent to form a polymer solution. Particles of a cathode activematerial (e.g. selenium-carbon hybrid particles, selenium-graphitehybrid particles, selenium-graphene hybrid particles, selenium compoundparticles, metal selenide particles, etc.) can be dispersed in thispolymer solution to form a suspension (dispersion or slurry) of anactive material particle-polymer (monomer or oligomer) mixture. Thissuspension can then be subjected to a solvent removal treatment whileindividual particles remain substantially separated from one another.The polymer (or monomer or oligomer) precipitates out to deposit onsurfaces of these active material particles. This can be accomplished,for instance, via spray drying, ultrasonic spraying, air-assistedspraying, aerosolization, and other secondary particle formationprocedures.

It is essential for these materials to form a lightly cross-linkednetwork of polymer chains. In other words, the network polymer orcross-linked polymer should have a relatively low degree ofcross-linking or low cross-link density to impart a high elasticdeformation.

The cross-link density of a cross-linked network of polymer chains maybe defined as the inverse of the molecular weight between cross-links(Mc). The cross-link density can be determined by the equation,Mc=ρRT/Ge, where Ge is the equilibrium modulus as deter mined by atemperature sweep in dynamic mechanical analysis, p is the physicaldensity, R is the universal gas constant in J/mol*K and T is absolutetemperature in K. Once Ge and p are determined experimentally, then Mcand the cross-link density can be calculated.

The magnitude of Mc may be normalized by dividing the Mc value by themolecular weight of the characteristic repeat unit in the cross-linkchain or chain linkage to obtain a number, Nc, which is the number ofrepeating units between two cross-link points. We have found that theelastic deformation strain correlates very well with Mc and Nc. Theelasticity of a cross-linked polymer derives from a large number ofrepeating units (large Nc) between cross-links. The repeating units canassume a more relax conformation (e.g. random coil) when the polymer isnot stressed. However, when the polymer is mechanically stressed, thelinkage chain uncoils or gets stretched to provide a large deformation.A long chain linkage between cross-link points (larger Nc) enables alarger elastic deformation. Upon release of the load, the linkage chainreturns to the more relaxed or coiled state. During mechanical loadingof a polymer, the cross-links prevent slippage of chains that otherwiseform plastic deformation (non-recoverable).

Preferably, the Nc value in a elastomer is greater than 5, morepreferably greater than 10, further more preferably greater than 100,and even more preferably greater than 200. These Nc values can bereadily controlled and varied to achieve different elastic deformationvalues by using different cross-linking agents with differentfunctionalities, and by designing the polymerization and cross-linkingreactions to proceed at different temperatures for different periods oftime.

Alternatively, Mooney-Rilvin method may be used to determine the degreeof cross-linking. Crosslinking also can be measured by swellingexperiments. In a swelling experiment, the crosslinked sample is placedinto a good solvent for the corresponding linear polymer at a specifictemperature, and either the change in mass or the change in volume ismeasured. The higher the degree of crosslinking, the less swelling isattainable. Based on the degree of swelling, the Flory InteractionParameter (which relates the solvent interaction with the sample, FloryHuggins Eq.), and the density of the solvent, the theoretical degree ofcrosslinking can be calculated according to Flory's Network Theory. TheFlory-Rehner Equation can be useful in the determination ofcross-linking. The elastomer for encapsulation may contain asimultaneous interpenetrating network (SIN) polymer, wherein twocross-linking chains intertwine with each other, or asemi-interpenetrating network polymer (semi-IPN), which contains across-linked polymer and a linear polymer.

The elastomer may form a mixture or blend with an electron-conductingpolymer selected from polyaniline, polypyrrole, polythiophene,polyfuran, a bi-cyclic polymer, derivatives thereof (e.g. sulfonatedversions), or a combination thereof.

In some embodiments, the elastomer may form a mixture with a lithiumion-conducting polymer selected from poly(ethylene oxide) (PEO),polypropylene oxide (PPO), poly(acrylonitrile) (PAN), poly(methylmethacrylate) (PMMA), poly(vinylidene fluoride) (PVDF), poly bis-methoxyethoxyethoxide-phosphazene, polyvinyl chloride, polydimethylsiloxane,poly(vinylidene fluoride)-hexafluoropropylene (PVDF-HFP), a derivativethereof (e.g. sulfonated versions), or a combination thereof.

Some elastomers are saturated rubbers that cannot be cured by sulfurvulcanization; they are made into a rubbery or elastomeric material viadifferent means: e.g. by having a copolymer domain that holds otherlinear chains together. Each of these elastomers can be used toencapsulate particles of an anode active material by one of severalmeans: melt mixing (followed by pelletizing and ball-milling, forinstance), solution mixing (dissolving the anode active materialparticles in an uncured polymer, monomer, or oligomer, with or withoutan organic solvent) followed by drying (e.g. spray drying), interfacialpolymerization, or in situ polymerization of elastomer in the presenceof anode active material particles.

Saturated rubbers and related elastomers in this category include EPM(ethylene propylene rubber, a copolymer of ethylene and propylene), EPDMrubber (ethylene propylene diene rubber, a terpolymer of ethylene,propylene and a diene-component), epichlorohydrin rubber (ECO),polyacrylic rubber (ACM, ABR), silicone rubber (SI, Q, VMQ),fluorosilicone rubber (FVMQ), fluoroelastomers (FKM, and FEPM; such asViton, Tecnoflon, Fluorel, Aflas and Dai-El), perfluoroelastomers (FFKM:Tecnoflon PFR, Kalrez, Chemraz, Perlast), polyether block amides (PEBA),chlorosulfonated polyethylene (CSM; e.g. Hypalon), and ethylene-vinylacetate (EVA), thermoplastic elastomers (TPE), protein resilin, andprotein elastin. Polyurethane and its copolymers (e.g. urea-urethanecopolymer) are particularly useful elastomeric shell materials forencapsulating active material particles.

C. Encapsulation of Cathode Active Material Particles by an Elastomer

Several micro-encapsulation processes require the elastomer or itsprecursor (monomer or oligomer) to be dissolvable in a solvent.Fortunately, all the elastomers or their precursors used herein aresoluble in some common solvents. The un-cured polymer or its precursorcan be readily dissolved in a common organic solvent to form a solution.This solution can then be used to encapsulate solid particles viaseveral of the micro-encapsulation methods to be discussed in whatfollows. Upon encapsulation, the polymer shell is then polymerized andcross-linked.

There are three broad categories of micro-encapsulation methods that canbe implemented to produce elastomer-encapsulated particles of a cathodeactive material: physical methods, physico-chemical methods, andchemical methods. The physical methods include pan-coating,air-suspension coating, centrifugal extrusion, vibration nozzle, andspray-drying methods. The physico-chemical methods include ionotropicgelation and coacervation-phase separation methods. The chemical methodsinclude interfacial polycondensation, interfacial cross-linking, in-situpolymerization, and matrix polymerization.

Pan-coating method: The pan coating process involves tumbling the activematerial particles in a pan or a similar device while the encapsulatingmaterial (e.g. monomer/oligomer, polymer melt, polymer/solvent solution)is applied slowly until a desired encapsulating shell thickness isattained.

Air-suspension coating method: In the air suspension coating process,the solid particles (core material) are dispersed into the supportingair stream in an encapsulating chamber. A controlled stream of apolymer-solvent solution (polymer or its monomer or oligomer dissolvedin a solvent; or its monomer or oligomer alone in a liquid state) isconcurrently introduced into this chamber, allowing the solution to hitand coat the suspended particles. These suspended particles areencapsulated (fully coated) with a polymer or its precursor moleculeswhile the volatile solvent is removed, leaving a very thin layer ofpolymer (or its precursor, which is cured/hardened subsequently) onsurfaces of these particles. This process may be repeated several timesuntil the required parameters, such as full-coating thickness (i.e.encapsulating shell or wall thickness), are achieved. The air streamwhich supports the particles also helps to dry them, and the rate ofdrying is directly proportional to the temperature of the air stream,which can be adjusted for optimized shell thickness.

In a preferred mode, the particles in the encapsulating zone portion maybe subjected to re-circulation for repeated coating. Preferably, theencapsulating chamber is arranged such that the particles pass upwardsthrough the encapsulating zone, then are dispersed into slower movingair and sink back to the base of the encapsulating chamber, enablingrepeated passes of the particles through the encapsulating zone untilthe desired encapsulating shell thickness is achieved.

Centrifugal extrusion: Active material particles may be encapsulatedusing a rotating extrusion head containing concentric nozzles. In thisprocess, a stream of core fluid (slurry containing particles of amaterial dispersed in a solvent) is surrounded by a sheath of shellsolution or melt. As the device rotates and the stream moves through theair it breaks, due to Rayleigh instability, into droplets of core, eachcoated with the shell solution. While the droplets are in flight, themolten shell may be hardened or the solvent may be evaporated from theshell solution. If needed, the capsules can be hardened after formationby catching them in a hardening bath. Since the drops are formed by thebreakup of a liquid stream, the process is only suitable for liquid orslurry. A high production rate can be achieved. Up to 22.5 kg ofmicrocapsules can be produced per nozzle per hour and extrusion headscontaining 16 nozzles are readily available.

Vibrational nozzle method: Core-shell encapsulation ormatrix-encapsulation of an active material can be conducted using alaminar flow through a nozzle and vibration of the nozzle or the liquid.The vibration has to be done in resonance with the Rayleigh instability,leading to very uniform droplets. The liquid can consist of any liquidswith limited viscosities (1-50,000 mPa·s): emulsions, suspensions orslurry containing the active material. The solidification can be doneaccording to the used gelation system with an internal gelation (e.g.sol-gel processing, melt) or an external (additional binder system, e.g.in a slurry).

Spray-drying: Spray drying may be used to encapsulate particles of anactive material when the active material is dissolved or suspended in amelt or polymer solution. In spray drying, the liquid feed (solution orsuspension) is atomized to form droplets which, upon contacts with hotgas, allow solvent to get vaporized and thin polymer shell to fullyembrace the solid particles of the active material.

Coacervation-phase separation: This process consists of three stepscarried out under continuous agitation:

-   (a) Formation of three immiscible chemical phases: liquid    manufacturing vehicle phase, core material phase and encapsulation    material phase. The core material is dispersed in a solution of the    encapsulating polymer (or its monomer or oligomer). The    encapsulating material phase, which is an immiscible polymer in    liquid state, is formed by (i) changing temperature in polymer    solution, (ii) addition of salt, (iii) addition of non-solvent,    or (iv) addition of an incompatible polymer in the polymer solution.-   (b) Deposition of encapsulation shell material: core material being    dispersed in the encapsulating polymer solution, encapsulating    polymer material coated around core particles, and deposition of    liquid polymer embracing around core particles by polymer adsorbed    at the interface formed between core material and vehicle phase; and-   (c) Hardening of encapsulating shell material: shell material being    immiscible in vehicle phase and made rigid via thermal,    cross-linking, or dissolution techniques.

Interfacial polycondensation and interfacial cross-linking: Interfacialpolycondensation entails introducing the two reactants to meet at theinterface where they react with each other. This is based on the conceptof the Schotten-Baumann reaction between an acid chloride and a compoundcontaining an active hydrogen atom (such as an amine or alcohol),polyester, polyurea, polyurethane, or urea-urethane condensation. Underproper conditions, thin flexible encapsulating shell (wall) formsrapidly at the interface. A solution of the active material and a diacidchloride are emulsified in water and an aqueous solution containing anamine and a polyfunctional isocyanate is added. A base may be added toneutralize the acid formed during the reaction. Condensed polymer shellsform instantaneously at the interface of the emulsion droplets.Interfacial cross-linking is derived from interfacial polycondensation,wherein cross-linking occurs between growing polymer chains and amulti-functional chemical groups to form an elastomer shell material.

In-situ polymerization: In some micro-encapsulation processes, activematerials particles are fully coated with a monomer or oligomer first.Then, direct polymerization and cross-linking of the monomer or oligomeris carried out on the surfaces of these material particles.

Matrix polymerization: This method involves dispersing and embedding acore material in a polymeric matrix during formation of the particles.This can be accomplished via spray-drying, in which the particles areformed by evaporation of the solvent from the matrix material.

Another possible route is the notion that the solidification of thematrix is caused by a chemical change.

D. Additional Details about the Encapsulated Particulates, the CathodeLayer, and the Structure of Li—Se, Na—Se, and K—Se Cells

The anode active material layer of an alkali metal-selenium cell cancontain a foil or coating of Li, Na, or K metal supported by a currentcollector (e.g. Cu foil), as illustrated in the left-hand portion ofFIG. 1(A) for a prior art Li—Se cell. Alternatively, the anode activematerial may contain, for instance, particles of prelithiated Siparticles or surface-stabilized Li particles, as illustrated in theleft-hand portion of FIG. 1(B). However, the cathode layer in theinstant cell is distinct, as already discussed above.

The electrolyte for an alkali metal-selenium cell may be an organicelectrolyte, ionic liquid electrolyte, gel polymer electrolyte,solid-state electrolyte (e.g. polymer solid electrolyte or inorganicsolid electrolyte), quasi-solid electrolyte or a combination thereof.The electrolyte typically contains an alkali metal salt (lithium salt,sodium salt, and/or potassium salt) dissolved in a solvent.

The solvent may be selected from 1,3-dioxolane (DOL),1,2-dimethoxyethane (DME), tetraethylene glycol dimethylether (TEGDME),poly(ethylene glycol) dimethyl ether (PEGDME), diethylene glycol dibutylether (DEGDBE), 2-ethoxyethyl ether (EEE), sulfone, sulfolane, ethylenecarbonate (EC), dimethyl carbonate (DMC), methylethyl carbonate (MEC),diethyl carbonate (DEC), ethyl propionate, methyl propionate, propylenecarbonate (PC), gamma-butyrolactone (γ-BL), acetonitrile (AN), ethylacetate (EA), propyl formate (PF), methyl formate (MF), toluene, xylene,methyl acetate (MA), fluoroethylene carbonate (FEC), vinylene carbonate(VC), allyl ethyl carbonate (AEC), a hydrofluoroether, a roomtemperature ionic liquid solvent, or a combination thereof.

The electrolytic salts to be incorporated into a non-aqueous electrolytemay be selected from a lithium salt such as lithium perchlorate(LiClO₄), lithium hexafluorophosphate (LiPF₆), lithium borofluoride(LiBF₄), lithium hexafluoroarsenide (LiAsF₆), lithiumtrifluoro-metasulfonate (LiCF₃SO₃), bis-trifluoromethyl sulfonylimidelithium (LiN(CF₃SO₂)₂), lithium bis(oxalato)borate (LiBOB), lithiumoxalyldifluoroborate (LiBF₂C₂O₄), lithium oxalyldifluoroborate(LiBF₂C₂O₄), lithium nitrate (LiNO₃), Li-fluoroalkyl-phosphates(LiPF3(CF₂CF₃)₃), lithium bisperfluoroethysulfonylimide (LiBETI), anionic liquid salt, sodium perchlorate (NaClO₄), potassium perchlorate(KClO₄), sodium hexafluorophosphate (NaPF₆), potassiumhexafluorophosphate (KPF₆), sodium borofluoride (NaBF₄), potassiumborofluoride (KBF₄), sodium hexafluoroarsenide, potassiumhexafluoroarsenide, sodium trifluoro-metasulfonate (NaCF₃SO₃), potassiumtrifluoro-metasulfonate (KCF₃SO₃), bis-trifluoromethyl sulfonylimidesodium (NaN(CF₃SO₂)₂), sodium trifluoromethanesulfonimide (NaTFSI), andbis-trifluoromethyl sulfonylimide potassium (KN(CF₃SO₂)₂). Among them,LiPF₆, LiBF₄ and LiN(CF₃SO₂)₂ are preferred for Li—Se cells, NaPF₆ andLiBF₄ for Na—Se cells, and KBF₄ for K—Se cells. The content ofaforementioned electrolytic salts in the non-aqueous solvent ispreferably 0.5 to 3.0 M (mol/L) at the cathode side and 3.0 to >10 M atthe anode side.

The ionic liquid is composed of ions only. Ionic liquids are low meltingtemperature salts that are in a molten or liquid state when above adesired temperature. For instance, a salt is considered as an ionicliquid if its melting point is below 100° C. If the melting temperatureis equal to or lower than room temperature (25° C.), the salt isreferred to as a room temperature ionic liquid (RTIL). The IL salts arecharacterized by weak interactions, due to the combination of a largecation and a charge-delocalized anion. This results in a low tendency tocrystallize due to flexibility (anion) and asymmetry (cation).

A typical and well-known ionic liquid is formed by the combination of a1-ethyl-3-methylimidazolium (EMI) cation and anN,N-bis(trifluoromethane)sulphonamide (TFSI) anion. This combinationgives a fluid with an ionic conductivity comparable to many organicelectrolyte solutions and a low decomposition propensity and low vaporpressure up to −300-400° C. This implies a generally low volatility andnon-flammability and, hence, a much safer electrolyte for batteries.

Ionic liquids are basically composed of organic ions that come in anessentially unlimited number of structural variations owing to thepreparation ease of a large variety of their components. Thus, variouskinds of salts can be used to design the ionic liquid that has thedesired properties for a given application. These include, among others,imidazolium, pyrrolidinium and quaternary ammonium salts as cations andbis(trifluoromethanesulphonyl) imide, bis(fluorosulphonyl)imide, andhexafluorophosphate as anions. Based on their compositions, ionicliquids come in different classes that basically include aprotic, proticand zwitterionic types, each one suitable for a specific application.

Common cations of room temperature ionic liquids (RTILs) include, butnot limited to, tetraalkylammonium, di-, tri-, andtetra-alkylimidazolium, alkylpyridinium, dialkyl-pyrrolidinium,dialkylpiperidinium, tetraalkylphosphonium, and trialkylsulfonium.Common anions of RTILs include, but not limited to, BF₄ ⁻, B(CN)₄ ⁻,CH₃BF₃ ⁻, CH2CHBF₃ ⁻, CF₃BF₃ ⁻, C₂F₅BF₃ ⁻, n-C₃F₇BF₃ ⁻, n-C₄F₉BF₃ ⁻, PF₆⁻, CF₃CO₂ ⁻, CF₃SO₃ ⁻, N(SO₂CF₃)₂ ⁻, N(COCF₃)(SO₂CF₃)⁻, N(SO₂F)₂ ⁻,N(CN)₂ ⁻, C(CN)₃ ⁻, SCN⁻, SeCN⁻, CuCl₂ ⁻, AlCl₄ ⁻, F(HF)_(2.3) ⁻, etc.Relatively speaking, the combination of imidazolium- or sulfonium-basedcations and complex halide anions such as AlCl₄ ⁻, BF₄ ⁻, CF₃CO₂ ⁻,CF₃SO₃ ⁻, NTf₂ ⁻, N(SO₂F)₂ ⁻, or F(HF)_(2.3) ⁻ results in RTILs withgood working conductivities.

RTILs can possess archetypical properties such as high intrinsic ionicconductivity, high thermal stability, low volatility, low (practicallyzero) vapor pressure, non-flammability, the ability to remain liquid ata wide range of temperatures above and below room temperature, highpolarity, high viscosity, and wide electrochemical windows. Theseproperties, except for the high viscosity, are desirable attributes whenit comes to using an RTIL as an electrolyte ingredient (a salt and/or asolvent) in a Li—Se cell.

In the presently invented products (including the alkali metal cell, thecathode active layer, and the cathode active material powder), the corematerial (to be encapsulated by a thin layer of elastomer) contains theselenium-carbon hybrid, selenium-graphite hybrid, selenium-graphenehybrid, conducting polymer-selenium hybrid, metal selenide, seleniumcompound, etc. These hybrid or compound materials are produced in theform of particles that contain a mixture, blend, composite, or bondedentity of selenium or selenide with a carbon, graphite, graphene, orconducting polymer material. Metal selenides (e.g. lithium polyselenide,sodium polyselenide, etc.) and selenium compounds are readily availablein a fine particle form. Selenium can be combined with a conductingmaterial (carbon, graphite, graphene, and/or conducting polymer) to forma composite, mixture, or bonded entity (e.g. selenium bonded on grapheneoxide surface).

There are many well-known procedures that can be used to make theaforementioned selenium-containing materials into particles. Forinstance, one may mix solid selenium with a carbon or graphite materialto form composite particles using ball-milling. The resulting particlesare typically ellipsoidal or potato-like in shape having a size from 1to 20 μm. Also, one may infiltrate Se or selenide into the pores ofporous carbon or graphite particles (e.g. activated carbon, mesoporouscarbon, activated carbon fibers, etc.) using vapor phase infiltration,solution infiltration, chemical infiltration, or electrochemicalinfiltration. Alternatively, one may deposit selenium onto surfaces ofgraphene sheets, CNTs, carbon nanofibers, etc. and then form theseSe-coated nanomaterials into a spherical or ellipsoidal shape usinghigh-intensity ball-milling, spray-drying (of their suspensions),aerosol formation, etc. These particles are then encapsulated with anelastomer using the micro-encapsulation processes discussed above.

The cathode in a conventional Li—Se cell typically has less than 70% byweight of selenium in a composite cathode composed of selenium and theconductive additive/support. Even when the selenium content in the priorart composite cathode reaches or exceeds 70% by weight, the specificcapacity of the composite cathode is typically significantly lower thanwhat is expected based on theoretical predictions. For instance, thetheoretical specific capacity of selenium is 675 mAh/g. A compositecathode composed of 70% selenium (Se) and 30% carbon black (CB), withoutany binder, should be capable of storing up to 675×70%=472 mAh/g.Unfortunately, the observed specific capacity is typically less than 75%of 472 mAh/g or 354 mAh/g (often less than 50% or 236 mAh/g in thisexample) of what could be achieved. In other words, the active material(Se) utilization rate is typically less than 75% (or even <50%). Thishas been a major issue in the art of Li—Se cells and there has been nosolution to this problem.

Thus, it is highly advantageous to obtain a high selenium loading andyet, concurrently, maintaining an ultra-thin/small thickness/diameter ofselenium for significantly enhanced selenium utilization efficiency,energy density and power density. For instance, one can depositnanoscaled selenium (1-5 nm thick) on graphene surfaces using chemical,electrochemical, or vapor deposition to form Se-coated or Se-bondedgraphene sheets. These Se-coated or Se-bonded graphene sheets are thenaggregated together using a tumbling mixing, ball-milling, or sprayingprocedure. These steps enable the preparation of Se-conducting materialhybrids that contain 85%-99% by weight selenium, yet maintaining acoating thickness or particle diameter from 1 nm to 5 nm. Thisultra-small dimension enables fast lithium diffusion andlithium-selenium reactions, leading to high Se utilization efficiency(hence, high energy density) even at high charge-discharge rates. Byimplementing a elastomer around these hybrid particles or seleniumcompound/selenide particles, we have significantly reduced and eveneliminated the shuttling effect, resulting in an alkali metal batterythat has long cycle-life.

Again, the shuttling effect is related to the tendency for selenium oralkali metal polyselenide that forms at the cathode to get dissolved inthe solvent and for the dissolved lithium polyselenide species tomigrate from the cathode to the anode, where they irreversibly reactwith lithium to form species that prevent selenide from returning backto the cathode during the subsequent discharge operation of the Li—Secell (the detrimental shuttling effect). It appears that the embracingelastomer has effectively trapped selenium and metal polyselenidetherein, thereby preventing or reducing such a dissolution and migrationissue. We have solved the most critical, long-standing problem of alkalimetal-selenium batteries.

The anode active material may contain, as an example, lithium metal foilor a high-capacity Si, Sn, or SnO₂ capable of storing a great amount oflithium. The cathode active material may contain pure selenium (if theanode active material contains lithium), lithium polyselenide, or anyselenium containing compound, molecule, or polymer. If the cathodeactive material includes lithium-containing species (e.g. lithiumpolyselenide) when the cell is made, the anode active material can beany material capable of storing a large amount of lithium (e.g. Si, Ge,Sn, SnO₂, etc.).

At the anode side, when lithium metal is used as the sole anode activematerial in a Li—Se cell, there is concern about the formation oflithium dendrites, which could lead to internal shorting and thermalrunaway. Herein, we have used two approaches, separately or incombination, to address this dendrite formation issue: one involving theuse of a high-concentration electrolyte at the anode side and the otherthe use of a nanostructure composed of conductive nanofilaments. For thelatter, multiple conductive nanofilaments are processed to form anintegrated aggregate structure, preferably in the form of a closelypacked web, mat, or paper, characterized in that these filaments areintersected, overlapped, or somehow bonded (e.g., using a bindermaterial) to one another to form a network of electron-conducting paths.The integrated structure has substantially interconnected pores toaccommodate electrolyte. The nanofilament may be selected from, asexamples, a carbon nanofiber (CNF), graphite nanofiber (GNF), carbonnanotube (CNT), metal nanowire (MNW), conductive nanofibers obtained byelectro-spinning, conductive electro-spun composite nanofibers,nanoscaled graphene platelet (NGP), or a combination thereof. Thenanofilaments may be bonded by a binder material selected from apolymer, coal tar pitch, petroleum pitch, mesophase pitch, coke, or aderivative thereof.

Nanofibers may be selected from the group consisting of an electricallyconductive electro-spun polymer fiber, electro-spun polymernanocomposite fiber comprising a conductive filler, nanocarbon fiberobtained from carbonization of an electrospun polymer fiber,electro-spun pitch fiber, and combinations thereof. For instance, ananostructured electrode can be obtained by electro-spinning ofpolyacrylonitrile (PAN) into polymer nanofibers, followed bycarbonization of PAN. It may be noted that some of the pores in thestructure, as carbonized, are greater than 100 nm and some smaller than100 nm.

The presently invented cathode active layer may be incorporated in oneof at least four broad classes of rechargeable lithium metal cells (or,similarly, for sodium metal or potassium metal cells):

-   -   (A) Lithium metal-selenium with a conventional anode        configuration: The cell contains an optional cathode current        collector, a presently invented cathode layer, a        separator/electrolyte, and an anode current collector. Potential        dendrite formation may be overcome by using a high-concentration        electrolyte or solid-state electrolyte at the anode.    -   (B) Lithium metal-selenium cell with a nanostructured anode        configuration: The cell contains an optional cathode current        collector, a cathode herein invented, a separator/electrolyte,        an optional anode current collector, and a nanostructure to        accommodate lithium metal that is deposited back to the anode        during a charge or re-charge operation. This nanostructure (web,        mat, or paper) of nanofilaments provide a uniform electric field        enabling uniform Li metal deposition, reducing the propensity to        form dendrites. This configuration can provide a dendrite-free        cell for a long and safe cycling behavior.    -   (C) Lithium ion-selenium cell with a conventional anode: For        instance, the cell contains an anode composed of anode active        graphite particles bonded by a binder, such as polyvinylidene        fluoride (PVDF) or styrene-butadiene rubber (SBR). The cell also        contains a cathode current collector, a cathode of the instant        invention, a separator/electrolyte, and an anode current        collector; and    -   (D) Lithium ion-selenium cell with a nanostructured anode: For        instance, the cell contains a web of nanofibers coated with Si        coating or bonded with Si nanoparticles. The cell also contains        an optional cathode current collector, a cathode herein        invented, a separator/electrolyte, and an anode current        collector. This configuration provides an ultra-high capacity,        high energy density, and a safe and long cycle life.

In the lithium-ion selenium cell (e.g. as described in (C) and (D)above), the anode active material can be selected from a wide range ofhigh-capacity materials, including (a) silicon (Si), germanium (Ge), tin(Sn), lead (Pb), antimony (Sb), bismuth (Bi), zinc (Zn), aluminum (Al),nickel (Ni), cobalt (Co), manganese (Mn), titanium (Ti), iron (Fe), andcadmium (Cd), and lithiated versions thereof; (b) alloys orintermetallic compounds of Si, Ge, Sn, Pb, Sb, Bi, Zn, Al, or Cd withother elements, and lithiated versions thereof, wherein said alloys orcompounds are stoichiometric or non-stoichiometric; (c) oxides,carbides, nitrides, selenides, phosphides, selenides, and tellurides ofSi, Ge, Sn, Pb, Sb, Bi, Zn, Al, Fe, Ni, Co, Ti, Mn, or Cd, and theirmixtures or composites, and lithiated versions thereof; (d) salts andhydroxides of Sn and lithiated versions thereof; (e) carbon or graphitematerials and prelithiated versions thereof; and combinations thereof.Non-lithiated versions may be used if the cathode side contains lithiumpolyselenides or other lithium sources when the cell is made.

A possible lithium metal cell may be comprised of an anode currentcollector, an electrolyte phase (optionally but preferably supported bya porous separator, such as a porous polyethylene-polypropyleneco-polymer film), a cathode of the instant invention, and an optionalcathode collector.

For a sodium ion-selenium cell or potassium ion-selenium cell, the anodeactive material layer can contain an anode active material selected fromthe group consisting of: (a) Sodium- or potassium-doped silicon (Si),germanium (Ge), tin (Sn), lead (Pb), antimony (Sb), bismuth (Bi), zinc(Zn), aluminum (Al), titanium (Ti), cobalt (Co), nickel (Ni), manganese(Mn), cadmium (Cd), and mixtures thereof; (b) Sodium- orpotassium-containing alloys or intermetallic compounds of Si, Ge, Sn,Pb, Sb, Bi, Zn, Al, Ti, Co, Ni, Mn, Cd, and their mixtures; (c) Sodium-or potassium-containing oxides, carbides, nitrides, selenides,phosphides, selenides, tellurides, or antimonides of Si, Ge, Sn, Pb, Sb,Bi, Zn, Al, Fe, Ti, Co, Ni, Mn, Cd, and mixtures or composites thereof;(d) Sodium or potassium salts; (e) particles of graphite, hard carbon,soft carbon or carbon particles and pre-sodiated versions thereof(pre-doped or pre-loaded with Na), and combinations thereof.

The following examples are presented primarily for the purpose ofillustrating the best mode practice of the present invention and shouldnot be construed as limiting the scope of the present invention.

Example 1: Preparation of Se Nanoparticles from SeO₂ and Ascorbic Acid

The starting materials include SeO₂, ascorbic acid (Vc) andpolysaccharides (CTS and CMC, separately). The CTS is a water-solublechitosan having a 73.5% degree of deacetylation and viscosity-averagemolecular weight of 4200; and CMC is carboxymethyl cellulose having adegree of substitution of 0.8 and molecular weight of 110,000. Theaqueous solutions of the materials were obtained by, for instance,dissolving 0.4 g of SeO₂ in 150 mL of de-ionized water under vigorousstirring.

For the preparation of selenium nanoparticles, appropriate amounts ofpolysaccharides, such as CTS or CMC solutions, were mixed with seleniousacid solution (the aqueous solution of SeO₂), respectively.Subsequently, the ascorbic acid solution was added into the mixtures toinitiate the reaction. In the reaction solution, the typicalconcentrations of CTS, CMC, selenious acid and ascorbic acid were 0.04%,0.25%, 1×10⁻³ and 4×10⁻³M, respectively. No stirring was conductedexcept the initial mixing of the reactants. The selenious acid solutionswere converted from colorless to red gradually after the addition of theascorbic acid. The resulting product mixtures were then dried in avacuum oven to collect Se nanoparticle powders. The reactions may beaccelerated by using a slightly higher temperature (e.g. 80° C. insteadof room temperature) and/or assisted by ultrasonic treatment.

Example 2: Preparation of Se Nanoparticles and Graphene-Wrapped Se fromNa₂SeO₃ and GO

Hollow and solid Se nanospheres were produced from Na₂SeO₃ by varyingthe amount of cetyltrimethyl ammonium bromide (CTAB) in the reactionsystem. In a representative procedure, 0.025 mol of sodium selenite(Na₂SeO₃) and 0.05 mol of ascorbic acid were separately dissolved in 50mL mixed solution (Vwater/Vethanol=1:1) with the assistance of CTAB atambient temperature. After adding the ascorbic acid, the red solutionturned to brick red. The color phenomenon was due to the formation ofα-Se particles. After 18 h, the products were washed with water andabsolute ethanol. Subsequently the product changed progressively fromred to gray, indicating that the amorphous Se phase had transformed to atrigonal phase (t-Se). The content of CTAB could be changed to getdifferent morphologies of the nano Se.

Example 3: Preparation of Selenium Nanowires

Selenium nanowires were synthesized from SeO₂. In a typical reactionprocess, SeO₂ (0.25 g) and β-cyclodextrin (0.25 g) were added into aglass beaker containing 50 mL distilled water. The mixture was stirredfor about 10 min to give a clear solution, which was promptly pouredinto another glass beaker containing ascorbic acid solution (50 mL,0.028M) under continuous stirring. After reacting for 4 h, the productwas collected by centrifugation and washed with deionized water andabsolute ethanol several times. Then it was re-dispersed in ethanol andallowed to age for 2 h without stirring. Subsequently, the products weredried in a vacuum at 60° C. for 5 h to recover Se nanowires.

Example 4: Hydrothermal Synthesis of Se Nanowires from (NH₄)₂S₂O₃ andNa₂SeO₃

A low-temperature hydrothermal synthesis route was conducted for directproduction of crystalline trigonal selenium nanowires, using (NH₄)₂S₂O₃and Na₂SeO₃ as the starting materials in the presence of a surfactant,sodium dodecyl sulfate (SDS). In a typical procedure, equivalent molaramounts of (NH₄)₂S₂O₃ and Na₂SeO₃ (10 mmol) were added to an aqueoussolution (50 mL) of SDS (0.325 g). The solution was stirred forapproximately 20 min until the solids had completely dissolved, and a0.2 M homogeneous solution was formed. The solution was then transferredto a Teflon-lined autoclave having a capacity of 60 mL. The autoclavewas sealed and heated at 110° C. for 17 h. and then allowed to cool toroom temperature naturally over a period of about 5 h. The resultingprecipitate was rinsed with distilled water and absolute alcohol severaltimes. After drying in vacuo at 40° C. for 4 h, the orange-red. powderswere collected. The hydrothermal synthesis of t-Se nanowires may bedescribed by the following chemical reaction:

The product yield was approximately 95%.

Example 5: Preparation of Se Nanoplatelets

In a typical synthesis procedure, 1 mmol commercial Se powder and 20 mLethylenediamine were poured into a Teflon-lined autoclave with acapacity of 30 mL. The autoclave was sealed and maintained at 160° C.for 2 h and then cooled to room temperature to produce a brownhomogeneous solution. Subsequently, 100 mL acetone at −18° C. wasinjected into the brown homogeneous solution, and a brick-red mixturewas obtained. After aging the brick-red mixture for 24 hours at −18° C.,the precipitates were centrifuged, washed several times with distilledwater and absolute alcohol, and finally dried in air at 60° C. for 24 h.The powder was then subjected to ball-milling for 30-60 minutes toobtain Se nanoplatelets. Some of the Se nanoplatelets were poured into agraphene suspension obtained in Example 9 to make a slurry, which wasspray-dried to yield pristine graphene-wrapped Se nanoplatelets.

Example 6: Preparation of Tetragonal Selenium Nanowires and Nanotubes

In a typical procedure of synthesizing Se nanowires, 0.52 g Na₂SeO₃ and2 g glucose were dissolved in 320 mL water hosted in a 500 mL beaker.After mixing for 20 min under vigorous magnetic stirring, the beakercontaining the mixture solution was sealed and maintained in an oven at85° C. A hot turbid brick-red solution was obtained, indicating theamorphous selenium being generated. The hot solution was cooled down bycold water in order to quench the reaction. The product was collected bycentrifugation and washed several times with deionized water to removethe impurities. The final brick-red product was re-dispersed in 10 mLabsolute ethanol to form a dispersion in a glass bottle, and then sealedand stored in darkness for further growth of Se nanowires. After thisdispersion was aged for one week at room temperature, a sponge-likeblack-gray solid (containing Se nanowires) was formed at the bottom andthe color of upper solution changed to colorless transparent. Thesynthesis of Se nanotubes was conducted under different conditions: 1.03g Na₂SeO₃ and 3 g glucose were dissolved in 100 mL water hosted in a 250mL beaker. After the solution was under constant stirring for 20 min,the beaker containing the mixture solution was sealed and thenmaintained at 85° C. for 4 h in an oven.

Example 7: Mixing of Selenium with Carbon/Graphite Particles ViaBall-Milling to Form Selenium-Containing Particles

Selenium particles and particles of soft carbon (graphitizabledisordered carbon), natural graphite, mesophase carbon, expandedgraphite flakes, carbon nanofibers, and graphene sheets (0% to 95% byweight of Se in the resulting composite) were physically blended andthen subjected to ball milling for 2-24 hours to obtain Se-containingcomposite particles (typically in a ball or potato shape). Theparticles, having a typical size of 1-10 containing various Se contents,were then embraced with a thin layer of elastomer. Some of the resultingparticulates were then made into a layer of cathode.

Example 8: Simple Selenium Melt or Liquid Solution Mixing

One way to combine selenium with a conducting material (e.g.carbon/graphite particles) is to use a solution or melt mixing process.Highly porous activated carbon particles, chemically etched mesocarbonmicrobeads (activated MCMBs), and exfoliated graphite worms were mixedwith selenium melt at 222-230° C. (slightly above the melting point ofSe, 221° C.) for 10-60 minutes to obtain selenium-impregnated carbonparticles.

Example 9: Electrochemical Impregnation of Se in Various PorousCarbon/Graphite Particles

The electrochemical impregnation of S into pores of activated carbonfibers, activated carbon nanotubes, and activated artificial graphiteparticles was conducted by aggregating these particles/fibers into aloosely packed layer. In this approach, an anode, electrolyte, and alayer of such a loosely packed structure (serving as a cathode layer)are positioned in an external container outside of a lithium-seleniumcell. The needed apparatus is similar to an electro-plating system,which is well-known in the art.

In a typical procedure, a metal polyselenide (M_(x)Se_(y)) was dissolvedin a solvent (e.g. mixture of DOL/DME in a volume ratio from 1:3 to 3:1)to form an electrolyte solution. An amount of a lithium salt may beoptionally added, but this is not required for external electrochemicaldeposition. A wide variety of solvents can be utilized for this purposeand there is no theoretical limit to what type of solvents can be used;any solvent can be used provided that there is some solubility of themetal polyselenide in this desired solvent. A greater solubility wouldmean a larger amount of selenium can be derived from the electrolytesolution.

The electrolyte solution was then poured into a chamber or reactor undera dry and controlled atmosphere condition (e.g. He or nitrogen gas). Ametal foil was used as the anode and a layer of the porous structure asthe cathode; both being immersed in the electrolyte solution. Thisconfiguration constitutes an electrochemical impregnation and depositionsystem. The step of electrochemically impregnating selenium into poreswas conducted at a current density in the range of 1 mA/g to 10 A/g,based on the layer weight of the porous carbon/graphiteparticles/fibers.

The chemical reactions that occur in this reactor may be represented bythe following equation: M_(x)Se_(y)→M_(x)Se_(y−z)+zSe (typically z=1-4).The selenium coating thickness or particle diameter and the amount of Secoating/particles impregnated may be controlled by the electro-chemicalreaction current density, temperature and time. In general, a lowercurrent density and lower reaction temperature lead to a more uniformimpregnation of Se and the reactions are easier to control. A longerreaction time leads to a larger amount of Se saturated in the pores.Additionally, the electrochemical method is capable of rapidlyconverting the impregnated Se into metal polyselenide (lithiumpolyselenide, sodium polyselenide, and potassium polyselenide, etc.).

Example 10: Chemical Reaction-Induced Impregnation of Selenium

A chemical impregnation method was herein utilized to prepareSe-impregnated carbon fibers that have been chemically activated. Theprocedure began with adding 0.58 g Na₂Se into a flask that had beenfilled with 25 ml distilled water to form a Na₂Se solution. Then, 0.72 gelemental Se was suspended in the Na₂Se solution and stirred with amagnetic stirrer for about 2 hours at room temperature. The color of thesolution changed slowly to orange-yellow as the selenium dissolved.After dissolution of the selenium, a sodium polyselenide (Na₂Se_(x))solution was obtained where 4≤x≤10.

Subsequently, a selenium-impregnated carbon fiber sample was prepared bya chemical impregnation method in an aqueous solution. First, 180 mg ofexpansion-treated carbon fibers was suspended in 180 ml ultrapure waterwith a surfactant and then sonicated at 50° C. for 5 hours to form astable carbon fiber dispersion. Subsequently, the Na₂Se_(x) solution wasadded to the above-prepared dispersions in the presence of 5 wt %surfactant cetyl trimethyl-ammonium bromide (CTAB), the as-preparedcarbon fiber/Na₂Se_(x) blended solution was sonicated for another 2hours and then titrated into 100 ml of 2 mol/L HCOOH solution at a rateof 30-40 drops/min and stirred for 2 hours. Finally, the precipitate wasfiltered and washed with acetone and distilled water several times toeliminate salts and impurities. After filtration, the precipitate wasdried at 50° C. in a drying oven for 48 hours. The reaction may berepresented by the following reaction: Se_(x) ²⁻+2H⁺→(x−1) Se+H₂Se.

Example 11: Redox Chemical Reaction-Induced Impregnation of Selenium inActivated MCMBs and Activated Needle Coke

In this chemical reaction-based deposition process, sodium thiosulfate(Na₂Se₂O₃) was used as a selenium source and HCl as a reactant. Anactivated MCMB-water or activated needle coke-water suspension wasprepared and then the two reactants (HCl and Na₂Se₂O₃) were poured intothis suspension. The reaction was allowed to proceed at 25-75° C. for1-3 hours, leading to impregnation of S into pores of the activatedstructures. The reaction may be represented by the following reaction:2HCl+Na₂Se₂O₃→2NaCl+Se↓+SeO₂+H₂O.

Example 12: Elastomer-Encapsulated Se Nanoparticles, Nanowires, andNanoplatelets

Selenium nanoparticles, nanowires, and nanoplatelets were dispersed in apolymer solution (rubber prior to curing, dissolved in a liquid solvent)to form various suspensions or slurries. The micro-encapsulationtechniques discussed earlier (spray-drying, vibration-nozzle, andpan-coating, etc.) were used to produce powder masses (multipleparticulates) of elastomer-encapsulated Se nanoparticles, nanowires, andnanoplatelets. As examples, the following polymers were used: SBR,cis-polyisoprene, EPDM, polyurethane, and urethane-urea copolymer.

For electrochemical testing, as an example, the working electrodes wereprepared by mixing 85 wt. % active material (encapsulated particulatesor non-encapsulated Se particles, etc.), 7 wt. % acetylene black(Super-P), and 8 wt. % polyvinylidene fluoride (PVDF) binder dissolvedin N-methyl-2-pyrrolidinoe (NMP) to form a slurry of 5 wt. % total solidcontent. After coating the slurries on Al foil, the electrodes weredried at 120° C. in vacuum for 2 h to remove the solvent beforepressing. Then, the electrodes were cut into a disk (ϕ=12 mm) and driedat 100° C. for 24 h in vacuum. Electrochemical measurements were carriedout using CR2032 (3V) coin-type cells with lithium metal foil or sodiumfoil as the counter/reference electrode, Celgard 2400 membrane asseparator, and 1 M LiPF₆ electrolyte solution dissolved in a mixture ofethylene carbonate (EC) and diethyl carbonate (DEC) (EC-DEC, 1:1 v/v).The cell assembly was performed in an argon-filled glove-box. The CVmeasurements were carried out using a CH-6 electrochemical workstationat a scanning rate of 0.5-5.0 mV/s.

Example 13: Effect of Lithium Ion-Conducting Additive in an ElastomerShell

A wide variety of lithium ion-conducting additives were added to severaldifferent elastomer matrix materials to prepare encapsulation shellmaterials for protecting core particles of an anode active material. Wehave discovered that these elastomer composite materials are suitableencapsulation shell materials provided that their lithium ionconductivity at room temperature is no less than 10⁻⁷ S/cm. With thesematerials, lithium ions appear to be capable of readily diffusing in andout of the encapsulation shell having a thickness no greater than 1 μm.For thicker shells (e.g. 10 μm), a lithium ion conductivity at roomtemperature no less than 10⁻⁴ S/cm would be required.

TABLE 1 Lithium ion conductivity of various elastomer compositecompositions as a shell material for protecting anode active materialparticles. Lithium- Li-ion Sample conducting Elastomer conductivity No.additive (1-2 μm thick) (S/cm) E-1 Li₂CO₃ + 70-99% polyurethane 2.7 ×10⁻⁶ to (CH₂OCO₂Li)₂ 1.8 × 10⁻³ S/cm E-2 Li₂CO₃ + 65-99% polyisoprene6.1 × 10⁻⁶ to (CH₂OCO₂Li)₂ 3.6 × 10⁻⁴ S/cm E-3 Li₂CO₃ + 65-99% SBR 6.5 ×10⁻⁶ to (CH₂OCO₂Li)₂ 5.2 × 10⁻⁴ S/cm D-4 Li₂CO₃ + 70-99% urethane-urea7.4 × 10⁻⁷ to (CH₂OCO₂Li)₂ 4.3 × 10⁻⁴ S/cm D-5 Li₂CO₃ + 75-99%polybutadiene 8.7 × 10⁻⁶ to (CH₂OCO₂Li)₂ 3.6 × 10⁻³ S/cm B1 LiF + LiOH +80-99% chloroprene 8.7 × 10⁻⁷ to Li₂C₂O₄ rubber 2.1 × 10⁻⁴ S/cm B2 LiF +HCOLi 80-99% EPDM 2.1 × 10⁻⁶ to 8.6 × 10⁻⁴ S/cm B3 LiOH 70-99%polyurethane 2.8 × 10⁻⁵ to 1.2 × 10⁻³ S/cm B4 Li₂CO₃ 70-99% polyurethane4.4 × 10⁻⁵ to 3.9 × 10⁻³ S/cm B5 Li₂C₂O₄ 70-99% polyurethane 9.3 × 10⁻⁶to 7.7 × 10⁻⁴ S/cm B6 Li₂CO₃ + LiOH 70-99% polyurethane 1.4 × 10⁻⁵ to1.6 × 10⁻³ S/cm C1 LiClO₄ 70-99% urethane-urea 4.8 × 10⁻⁵ to 2.2 × 10⁻³S/cm C2 LiPF₆ 70-99% urethane-urea 2.4 × 10⁻⁵ to 8.2 × 10⁻⁴ S/cm C3LiBF₄ 70-99% urethane-urea 1.2 × 10⁻³ to 1.2 × 10⁻⁴ S/cm C4 LiBOB +LiNO₃ 70-99% urethane-urea 6.8 × 10⁻⁶ to 1.2 × 10⁻⁴ S/cm S1 Sulfonated85-99% SBR 6.3 × 10⁻⁶ to polyaniline 4.2 × 10⁻⁴ S/cm S2 Sulfonated SBR85-99% SBR 5.2 × 10⁻⁶ to 2.2 × 10⁻⁴ S/cm S3 Sulfonated PVDF 80-99%chlorosulfonated 3.3 × 10⁻⁶ to polyethylene (CS-PE) 2.8 × 10⁻⁴ S/cm S4Polyethylene 80-99% CS-PE 4.9 × 10⁻⁶ to oxide 3.7 × 10⁻⁴ S/cm

Example 14: Cycle Stability of Various Rechargeable Lithium-SeleniumBattery Cells

Several series of Li metal-selenium and Li-ion selenium cells wereprepared using the presently prepared cathode layers. The first seriesis a Li metal cell containing a copper foil as an anode currentcollector and the second series is also a Li metal cell having ananostructured anode of conductive filaments (based on electro-spuncarbon fibers or CNFs). The third series is a Li-ion cell having ananostructured anode of conductive filaments (based on electro-spuncarbon fibers coated with a thin layer of Si using CVD) plus a copperfoil current collector. The fourth series is a Li-ion cell having aprelithiated graphite-based anode active material as an example of themore conventional anode. We have found that after large numbers ofcharge/discharge cycles, the cells containing a nanostructured anodewere essentially dendrite-free.

Charge storage capacities were measured periodically and recorded as afunction of the number of cycles. The specific discharge capacity hereinreferred to is the total charge inserted into the cathode during thedischarge, per unit mass of the composite cathode (counting the weightsof cathode active material, conductive additive or support, binder, andany optional additive combined). The specific charge capacity refers tothe amount of charges per unit mass of the composite cathode. Thespecific energy and specific power values presented in this section arebased on the total cell weight. The morphological or micro-structuralchanges of selected samples after a desired number of repeated chargingand recharging cycles were observed using both transmission electronmicroscopy (TEM) and scanning electron microscopy (SEM).

The cycling behaviors of 3 cells prepared in Example 10 are shown inFIG. 5, which indicates that elastomer encapsulation of Se-basedparticles, with or without carbon coating, provides the most stablecycling response. Carbon coating alone does not help to improve cyclingstability by much.

Shown in FIG. 6 are the cycling behaviors of 2 Li—Se cells prepared inExample 9; one cell has a cathode containing particulates ofpolyurethane-encapsulated selenium-CNT composite balls and the othercell has a cathode containing particulates of un-protected selenium-CNTcomposite balls. The elastomer has imparted cycle stability to the Li—Secell in a most dramatic manner.

FIG. 7 shows the cycling behavior of two room-temperature Li—Se cell:one cell has a cathode containing particulates of urethane-ureacopolymer-encapsulated selenium-MCMB (activated) composite particles andthe other cell has a cathode containing particulates of un-protectedselenium-MCMB (activated) composite particles. Again, the elastomer hassignificantly improved the cycle stability of the Li—Se cell.

The above cycling stability data have clearly demonstrated that theshuttling effect commonly associated with Li—Se cells has beensignificantly reduced or essentially eliminated by the presentlyinvented elastomer encapsulation approach.

FIG. 8 shows the Ragone plots (cell power density vs. cell energydensity) of two Li metal-selenium cells: one featuring a cathode layercomposed of elastomer-encapsulated Se nanowires and the other a cathodeof carbon black-Se nanoparticles ball-milled together. FIG. 9 shows theRagone plots (cell power density vs. cell energy density) of 4 alkalimetal-selenium cells: Na—Se cell featuring elastomer/RGO-encapsulatedparticles of selenium, Na—Se cell featuring carbon-coated Se particles,K—Se cell featuring elastomer/RGO-encapsulated Se particles, and K—Secell featuring polyaniline-coated Se particles.

FIG. 8 and FIG. 9 indicate that the presence of a elastomer embracing aselenium-based cathode active material does not compromise the energydensity of an alkali metal-selenium cell based on the consideration thatthis polymer shell is normally less electron-conducting than a carboncoating and less ion-conducting than a liquid electrolyte. To thecontrary, the energy density of the cell actually can be improved usingthe presently invented elastomer encapsulation approach.

In lithium battery industry, it is a common practice to define the cyclelife of a battery as the number of charge-discharge cycles that thebattery suffers 20% decay in capacity based on the initial capacitymeasured after the required electrochemical formation. Summarized inTable 2 below are the cycle life data of a broad array of batteriesfeaturing presently invented elastomer-encapsulated selenium cathodeparticles vs. other types of cathode active materials.

TABLE 2 Cycle life data of various lithium secondary (rechargeable)batteries. Initial Cycle life Sample Type & % of anode capacity (No. ofID Protective means active material (mAh/g) cycles) CNF-1 SBR 80% by wt.Se + 532 1,766 encapsulation 7% CNF + 3% SBR + 5% binder + 5% CB CNF-2Carbon 80% by wt. Se + 520 154 encapsulation 7% CNF + 3% carbon + 5%binder + 5% CB AC-1 No encapsulation 70% Se + 15% 452 176 AC + 8%binder + 7% CB AC-2 Encapsulated by 70% Se + 15% 455 1,445 PolyurethaneAC + 3% polymer (75%) + ethylene mixture + 5% oxide (25%) binder + 7% CBGn-3 Polyisoprene 90% S 601 1,212 encapsulation (coated on graphenesheets) Gn-4 Carbon 90% Se 488 182 encapsulation (coated on graphenesheets) CB-1 No encapsulation 70% Se + 453 66 22% CB + 8% binder CB-2Urethan-urea 70% Se + 460 1578 copolymer 20% CB + 4% encapsulationco-polymer + 6% binder

The following observations can be made from the data of Table 2 and FIG.5-FIG. 9:

-   -   1) The presently invented approach enables the Li—Se, Na—Se, and        K—Se batteries to deliver high cycling stability.    -   2) The invented approach also leads to alkali metal-selenium        batteries having exceptional energy densities and power        densities. A cell-level energy density as high as 428 Wh/kg has        been achieved with Li—Se cells featuring a cathode active        material encapsulated by a elastomer. Also quite surprisingly,        the cell delivers a power density as high as 3318 W/kg, 4-5        times higher than the typical power density of lithium-ion        batteries and that of conventional Li—Se cells. This power        density improvement is very significant in light of the notion        that Li—Se cells, being conversion-type cells, operate on some        chemical reactions during charge/discharge and, hence, typically        would be expected to deliver very low power densities (typically        <<500 W/kg).    -   3) Similar advantageous features are also observed with Na—Se        cells and K—Se cells. This is evidenced by FIG. 9, which shows        the Ragone plots (cell power density vs. cell energy density) of        4 alkali metal-selenium cells:

In summary, the present invention provides an innovative, versatile, andsurprisingly effective platform materials technology that enables thedesign and manufacture of superior alkali metal-selenium rechargeablebatteries. The alkali metal-selenium cell featuring a cathode layercontaining particulates of selenium particles encapsulated by aelastomer exhibits a high cathode active material utilization rate, highspecific capacity, high specific energy, high power density, little orno shuttling effect, and long cycle life. When a nanostructured carbonfilament web is implemented at the anode to support a lithium film (e.g.foil), the lithium dendrite issue is also suppressed or eliminated.

1. A method of manufacturing a rechargeable alkali metal-selenium cell,said method comprising: (a) providing a cathode and an optional cathodecurrent collector to support said cathode; (b) providing an alkali metalanode and an optional anode current collector to support said anode; and(c) combining the anode and the cathode and adding an electrolyte incontact with the anode and the cathode to form said alkalimetal-selenium cell; wherein said cathode contains multiple particulatesof a selenium-containing material selected from selenium, aselenium-carbon hybrid, selenium-graphite hybrid, selenium-graphenehybrid, conducting polymer-selenium hybrid, a metal selenide, a Se alloyor mixture with Sn, Sb, Bi, S, or Te, a selenium compound, or acombination thereof and wherein at least one of said particulatescomprises one or a plurality of said selenium-containing materialparticles being embraced or encapsulated by a thin layer of an elastomerhaving a recoverable tensile strain from 5% to 1000% when measuredwithout an additive or reinforcement being present in said polymer, alithium ion conductivity no less than 10⁻⁷ S/cm at room temperature, anda thickness from 0.5 nm to 10 μm.
 2. The manufacturing method of claim1, wherein a separator is added to electrically separate the anode andthe cathode.
 3. The manufacturing method of claim 1, wherein saidselenium-containing material is selected from a selenium-carbon hybrid,selenium-graphite hybrid, selenium-graphene hybrid, conductingpolymer-selenium hybrid, a metal selenide, a Se alloy or mixture withSn, Sb, Bi, S, or Te, a selenium compound, or a combination thereof. 4.The manufacturing method of claim 3, wherein said selenium-carbonhybrid, selenium-graphite hybrid, selenium-graphene hybrid, orconducting polymer-selenium hybrid is a mixture, blend, composite,chemically or physically bonded entity of selenium or selenide with acarbon, graphite, graphene, or conducting polymer material.
 5. Therechargeable alkali metal-selenium cell of claim 1, wherein saidelastomer contains a material selected from natural polyisoprene,synthetic polyisoprene, polybutadiene, chloroprene rubber,polychloroprene, butyl rubber, styrene-butadiene rubber, nitrile rubber,ethylene propylene rubber, ethylene propylene diene rubber,epichlorohydrin rubber, polyacrylic rubber, silicone rubber,fluorosilicone rubber, perfluoroelastomers, polyether block amides,chlorosulfonated polyethylene, ethylene-vinyl acetate, thermoplasticelastomer, protein resilin, protein elastin, ethyleneoxide-epichlorohydrin copolymer, polyurethane, urethane-urea copolymer,or a combination thereof.
 6. The manufacturing method of claim 1,wherein said elastomer has a thickness from 1 nm to 100 nm.
 7. Themanufacturing method of claim 1, wherein said elastomer has a lithiumion conductivity or sodium ion conductivity from 1×10⁻⁵ S/cm to 5×10⁻²S/cm.
 8. The manufacturing method of claim 1, wherein said elastomer hasa recoverable tensile strain from 10% to 300%.
 9. The manufacturingmethod of claim 1, wherein said providing multiple particulates includesencapsulating or embracing said one or a plurality ofselenium-containing material particles with said thin layer of elastomerusing a procedure selected from pan coating, air suspension, centrifugalextrusion, vibrational nozzle, spray-drying, ultrasonic spraying,coacervation-phase separation, interfacial polycondensation, in-situpolymerization, matrix polymerization, or a combination thereof.
 10. Themanufacturing method of claim 1, wherein said providing multipleparticulates includes encapsulating or embracing said one or a pluralityof selenium-containing material particles with a mixture of saidelastomer with an electronically conductive polymer, a lithium-ionconducting material, a sodium ion-conducting material, a reinforcementmaterial, or a combination thereof.
 11. The manufacturing method ofclaim 10, wherein said lithium ion-conducting material is dispersed insaid high-elasticity polymer and is selected from Li₂CO₃, Li₂O, Li₂C₂O₄,LiOH, LiX, ROCO₂Li, HCOLi, ROLi, (ROCO₂Li)₂, (CH₂OCO₂Li)₂, Li₂S,Li_(x)SO_(y), or a combination thereof, wherein X═F, Cl, I, or Br, R=ahydrocarbon group, 0<x≤1 and 1≤y≤4.
 12. The manufacturing method ofclaim 10, wherein said lithium ion-conducting material is dispersed insaid high-elasticity polymer and is selected from lithium perchlorate(LiClO₄), lithium hexafluorophosphate (LiPF₆), lithium borofluoride(LiBF₄), lithium hexafluoroarsenide (LiAsF₆), lithiumtrifluoro-metasulfonate (LiCF₃SO₃), bis-trifluoromethyl sulfonylimidelithium (LiN(CF₃SO₂)₂), lithium bis(oxalato)borate (LiBOB), lithiumoxalyldifluoroborate (LiBF₂C₂O₄), lithium oxalyldifluoroborate(LiBF₂C₂O₄), lithium nitrate (LiNO₃), Li-fluoroalkyl-phosphate(LiPF₃(CF₂CF₃)₃), lithium bisperfluoro-ethysulfonylimide (LiBETI),lithium bis(trifluoromethanesulfonyl)imide, lithiumbis(fluorosulfonyl)imide, lithium trifluoromethanesulfonimide (LiTFSI),an ionic liquid-based lithium salt, or a combination thereof.
 13. Themanufacturing method of claim 10, wherein said sodium ion-conductingmaterial is dispersed in said high-elasticity polymer and is selectedfrom Na₂CO₃, Na₂O, Na₂C₂O₄, NaOH, NaX, ROCO₂Na, HCONa, RONa, (ROCO₂Na)₂,(CH₂OCO₂Na)₂, Na₂S, Na_(x)SO_(y), or a combination thereof, wherein X═F,Cl, I, or Br, R=a hydrocarbon group, 0<x≤1 and 1≤y≤4.
 14. Themanufacturing method of claim 10, wherein said sodium ion-conductingmaterial is dispersed in said high-elasticity polymer and is selectedfrom sodium perchlorate (NaClO₄), sodium hexafluorophosphate (NaPF₆),sodium borofluoride (NaBF₄), sodium hexafluoroarsenide (NaAsF₆), sodiumtrifluoro-metasulfonate (NaCF₃SO₃), bis-trifluoromethyl sulfonylimidesodium (NaN(CF₃SO₂)₂), sodium bis(oxalato)borate (NaBOB), sodiumoxalyldifluoroborate (NaBF₂C₂O₄), sodium oxalyldifluoroborate(NaBF₂C₂O₄), sodium nitrate (NaNO₃), Na-fluoroalkyl-phosphates(NaPF₃(CF₂CF₃)₃), sodium bisperfluoro-ethysulfonylimide (NaBETI), sodiumbis(trifluoromethanesulfonyl)imide, sodium bis(fluorosulfonyl)imide,sodium trifluoromethanesulfonimide (NaTFSI), an ionic liquid-basedsodium salt, or a combination thereof.
 15. The manufacturing method ofclaim 1, wherein said elastomer contains from 0.1% to 50% by weight of alithium ion-conducting additive or sodium ion-conducting additivedispersed therein, or contains therein from 0.1% by weight to 10% byweight of a reinforcement nanofilament selected from carbon nanotube,carbon nanofiber, graphene, or a combination thereof.
 16. Themanufacturing method of claim 1, wherein said metal selenide containsM_(x)Se_(y), wherein x is an integer from 1 to 3 and y is an integerfrom 1 to 10, and M is a metal element selected from an alkali metal, analkaline metal selected from Mg or Ca, a transition metal, a metal fromgroups 13 to 17 of the periodic table, or a combination thereof.
 17. Themanufacturing method of claim 16, wherein said metal element M isselected from Li, Na, K, Mg, Zn, Cu, Ti, Ni, Co, Fe, or Al.
 18. A methodof manufacturing a powder mass for a lithium-selenium battery cathode,said method comprising (a) dispersing particles of a selenium-containingmaterial in an elastomer solution to form a slurry, wherein saidselenium-containing material is selected from a selenium-carbon hybrid,selenium-graphite hybrid, selenium-graphene hybrid, conductingpolymer-selenium hybrid, metal selenide, a Se alloy or mixture with Sn,Sb, Bi, S, or Te, a selenium compound, or a combination thereof; and (b)dispensing and forming said slurry into said powder mass comprisingmultiple particulates of said selenium-containing material wherein atleast one of said particulates comprises one or a plurality ofselenium-containing material particles being embraced or encapsulated bya thin layer of an elastomer having a recoverable tensile strain no lessthan 5% when measured without an additive or reinforcement, a lithiumion conductivity no less than 10⁻⁷ S/cm at room temperature, and athickness from 0.5 nm to 10 μm.
 19. The manufacturing method of claim18, wherein said step of dispensing and forming comprises a procedureselected from pan coating, air suspension, centrifugal extrusion,vibrational nozzle, spray-drying, ultrasonic spraying,coacervation-phase separation, interfacial polycondensation, in-situpolymerization, matrix polymerization, or a combination thereof.
 20. Themanufacturing method of claim 18, wherein said elastomer contains amaterial selected from natural polyisoprene, synthetic polyisoprene,polybutadiene, chloroprene rubber, polychloroprene, butyl rubber,styrene-butadiene rubber, nitrile rubber, ethylene propylene rubber,ethylene propylene diene rubber, epichlorohydrin rubber, polyacrylicrubber, silicone rubber, fluorosilicone rubber, perfluoroelastomers,polyether block amides, chlorosulfonated polyethylene, ethylene-vinylacetate, thermoplastic elastomer, protein resilin, protein elastin,ethylene oxide-epichlorohydrin copolymer, polyurethane, urethane-ureacopolymer, or a combination thereof.